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.