A new abstract class can be used to implement job function classes. It
handles the necessary logic for starting and stopping jobs, including
exception handling and rescheduling of recurring jobs.

This commit also includes the migration of data source jobs to the new
framework.

Using the 'import_string()' utility from Django allows the job script
class to be simplified, as module imports no longer need to avoid loops.
This should make it easier to queue and maintain jobs.

Instead of maintaining two separate job execution logics, the same job
is now used for both background and interactive execution.

The independent implementations of interactive and background script
execution have been merged into a single BackgroundJob implementation.

A new abstract class can be used to implement job function classes that
specialize in scheduling. These use the same logic as regular
BackgroundJobs, but ensure that they are only scheduled once at any given
time.

A new abstract class can be used to implement job function classes that
specialize in system background tasks (e.g. synchronization or
housekeeping). In addition to the features of the BackgroundJob and
ScheduledJob classes, these implement additional logic to not need to be
bound to an existing NetBox object and to setup job schedules on plugin
load instead of an interactive request.

This partially reverts commit db591d4. The 'run_now' parameter of
'enqueue()' remains, as its being used by following commits.

Because scripts are already linked through the Job Instance field, the
name is displayed twice. Removing this reduces redundancy and opens up
the possibility of simplifying the BackgroundJob framework in future
commits.

Instead of using separate classes, the logic of ScheduledJob is now
merged into the generic BackgroundJob class. This allows reusing the
same logic, but dynamically deciding whether to enqueue the same job
once or multiple times.

Instead of defining individual names on enqueue, BackgroundJob classes
can now set a job name in their meta class. This is equivalent to other
Django classes and NetBox scripts.

Unfortunately, work on this branch was done twice. However, most of the
commits affected different sections of the code. This merge commit
combines both efforts, but removes duplicate work.

ChoiceSet was moved to core in 40572b5.

System jobs usually perform low-priority background tasks and therefore
can use a different queue than 'default', which is used for regular jobs
related to specific objects.

As the job's name is set by enqueue(), it must not be passed in handle()
to avoid duplicate kwargs with the same name.

Not only can a job's interval change, but so can the time at which it is
scheduled to run. If a specific scheduled time is set, it will also be
checked against the current job schedule. If there are any changes, the
job is rescheduled with the new time.

Instead of using a class method for run(), a regular method is used for
this purpose. This gives the possibility to add more convenience methods
in the future, e.g. for interacting with the job object or for logging,
as implemented for scripts.

This reverts commit b17b205.

This commit reverts commits 4880d81 and 0b15ecf. Using the database
'connection_created' signal for job registration feels a little wrong at
this point, as it would trigger registration very often. However, the
background job framework is prepared for this use case and can be used
by plugins once the auto-registration of jobs is solved.

Defining names for background jobs was disabled with fb75389. The
preceeding changes in 257976d did forget the management command.