Rename GoodJob::Job to GoodJob::Execution

[bensheldon]

Closes #376.

• Preserves but deprecates GoodJob::Job.
• Also renames GoodJob::CurrentExecution to GoodJob::CurrentThread
• Updates Dashboard controllers and views/partials; puts the "Active Job ID" column first in the table view; wraps UUIDs with <code> tags
• Renames scopesGoodJob::Execution.with_job_class(class_name) to GoodJob::Execution.job_class(class_name)

[morgoth]

I'm wondering how many people are using internals of the gem.
In example we have some hand crafted simple monitoring tool in place to check for jobs, so we're using GoodJob::Job directly, but we also have tests for it so we will notice the change.

The other example is the hirefire gem (for autoscalling heroku dynos) that uses the class directly https://github.com/hirefire/hirefire-resource/blob/master/lib/hirefire/macro/good_job.rb

Do you plan to release it as 2.2 version? Maybe we should deprecate the Job somehow first?

[bensheldon]

@morgoth thanks for drawing my attention to that. You're right, there should be a deprecation notice on this, and probably preserve the presence of the GoodJob::Job class until v3.

[bensheldon]

I'm now preserving/deprecating GoodJob::Job instead of removing it (class Job < Execution).