Closes #18287: Enable periodic synchronization for data sources

[jeremystretch]

Closes: #18287

• Add sync_interval field to DataSource
• Automatically enqueue a SyncDataSourceJob when the configured interval for a DataSource changes

[jeremystretch]

I've noticed an issue that occurs when re-scheduling a job, but I don't think it's specific to this change. When enqueuing a new job after modifying the sync interval, the new job fill fail with an error like the following:

IntegrityError('duplicate key value violates unique constraint "core_job_job_id_key"\nDETAIL: Key (job_id)=(1f4fdacb-1825-451e-b324-05a2f7d2de66) already exists.')

I've seen this pop in other scenarios; I believe it's related to the background logic responsible for enqueuing Redis tasks. Will need to dig into this further.

[jnovinger]

No issues with the code, but ran in to what seem to be edge cases while testing the post-save signal handler.

[jnovinger]

💥 all of my edge cases are resolved!

Except for the one other case I just now noticed, which may or may not be worth addressing. I'd call it non-blocking.