Fix migrations issue with 'branch' column

[PavelSafronov]

Fix migrations issue "blob/text column 'branch' used in key specification without a key length" by changing the 'branch' column from TextField to CharField.

Original error trace:

root@226502d0e465:/source# nautobot-server migrate
System check identified some issues:

WARNINGS:
extras.Job.slug: (mysql.W003) MySQL may not allow unique CharFields to have a max_length > 255.
	HINT: See: https://docs.djangoproject.com/en/3.2/ref/databases/#mysql-character-fields
Operations to perform:
  Apply all migrations: admin, auth, circuits, contenttypes, database, dcim, django_celery_beat, django_rq, extras, ipam, nautobot_version_control, sessions, social_django, taggit, tenancy, users, virtualization
Running migrations:
  Applying nautobot_version_control.0002_branchmeta...Traceback (most recent call last):
  File "/usr/local/lib/python3.7/site-packages/django/db/backends/utils.py", line 82, in _execute
    return self.cursor.execute(sql)
  File "/usr/local/lib/python3.7/site-packages/django/db/backends/mysql/base.py", line 73, in execute
    return self.cursor.execute(query, args)
  File "/usr/local/lib/python3.7/site-packages/MySQLdb/cursors.py", line 206, in execute
    res = self._query(query)
  File "/usr/local/lib/python3.7/site-packages/MySQLdb/cursors.py", line 319, in _query
    db.query(q)
  File "/usr/local/lib/python3.7/site-packages/MySQLdb/connections.py", line 254, in query
    _mysql.connection.query(self, query)
MySQLdb.OperationalError: (1105, "blob/text column 'branch' used in key specification without a key length")

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/usr/local/bin/nautobot-server", line 8, in <module>
    sys.exit(main())
  File "/usr/local/lib/python3.7/site-packages/nautobot/core/cli.py", line 61, in main
    initializer=_configure_settings,  # Called after defaults
  File "/usr/local/lib/python3.7/site-packages/nautobot/core/runner/runner.py", line 263, in run_app
    management.execute_from_command_line([runner_name, command] + command_args)
  File "/usr/local/lib/python3.7/site-packages/django/core/management/__init__.py", line 419, in execute_from_command_line
    utility.execute()
  File "/usr/local/lib/python3.7/site-packages/django/core/management/__init__.py", line 413, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File "/usr/local/lib/python3.7/site-packages/django/core/management/base.py", line 354, in run_from_argv
    self.execute(*args, **cmd_options)
  File "/usr/local/lib/python3.7/site-packages/django/core/management/base.py", line 398, in execute
    output = self.handle(*args, **options)
  File "/usr/local/lib/python3.7/site-packages/django/core/management/base.py", line 89, in wrapped
    res = handle_func(*args, **kwargs)
  File "/usr/local/lib/python3.7/site-packages/django/core/management/commands/migrate.py", line 246, in handle
    fake_initial=fake_initial,
  File "/usr/local/lib/python3.7/site-packages/django/db/migrations/executor.py", line 117, in migrate
    state = self._migrate_all_forwards(state, plan, full_plan, fake=fake, fake_initial=fake_initial)
  File "/usr/local/lib/python3.7/site-packages/django/db/migrations/executor.py", line 147, in _migrate_all_forwards
    state = self.apply_migration(state, migration, fake=fake, fake_initial=fake_initial)
  File "/usr/local/lib/python3.7/site-packages/django/db/migrations/executor.py", line 227, in apply_migration
    state = migration.apply(state, schema_editor)
  File "/usr/local/lib/python3.7/site-packages/django/db/migrations/migration.py", line 126, in apply
    operation.database_forwards(self.app_label, schema_editor, old_state, project_state)
  File "/usr/local/lib/python3.7/site-packages/django/db/migrations/operations/models.py", line 92, in database_forwards
    schema_editor.create_model(model)
  File "/usr/local/lib/python3.7/site-packages/django/db/backends/base/schema.py", line 345, in create_model
    self.execute(sql, params or None)
  File "/usr/local/lib/python3.7/site-packages/django/db/backends/base/schema.py", line 145, in execute
    cursor.execute(sql, params)
  File "/usr/local/lib/python3.7/site-packages/django/db/backends/utils.py", line 98, in execute
    return super().execute(sql, params)
  File "/usr/local/lib/python3.7/site-packages/cacheops/transaction.py", line 97, in execute
    result = self._no_monkey.execute(self, sql, params)
  File "/usr/local/lib/python3.7/site-packages/django/db/backends/utils.py", line 66, in execute
    return self._execute_with_wrappers(sql, params, many=False, executor=self._execute)
  File "/usr/local/lib/python3.7/site-packages/django/db/backends/utils.py", line 75, in _execute_with_wrappers
    return executor(sql, params, many, context)
  File "/usr/local/lib/python3.7/site-packages/django/db/backends/utils.py", line 84, in _execute
    return self.cursor.execute(sql, params)
  File "/usr/local/lib/python3.7/site-packages/django/db/utils.py", line 90, in __exit__
    raise dj_exc_value.with_traceback(traceback) from exc_value
  File "/usr/local/lib/python3.7/site-packages/django/db/backends/utils.py", line 82, in _execute
    return self.cursor.execute(sql)
  File "/usr/local/lib/python3.7/site-packages/django/db/backends/mysql/base.py", line 73, in execute
    return self.cursor.execute(query, args)
  File "/usr/local/lib/python3.7/site-packages/MySQLdb/cursors.py", line 206, in execute
    res = self._query(query)
  File "/usr/local/lib/python3.7/site-packages/MySQLdb/cursors.py", line 319, in _query
    db.query(q)
  File "/usr/local/lib/python3.7/site-packages/MySQLdb/connections.py", line 254, in query
    _mysql.connection.query(self, query)
django.db.utils.OperationalError: (1105, "blob/text column 'branch' used in key specification without a key length")
root@226502d0e465:/source#