Fix BackgroundTask migration

[dbnicholson]

There were a few problems here:

• Jobs have no fun attribute. It should be func.
• Since a forward code migration was provided but no reverse code migration, Django assumes the the migration is not reversible.
• job_storage was not being imported from where it's defined. That would make it difficult to mock in tests.
• The JSON encoded parameters weren't sorted, which would make them impossible to test.

A test is added to verify that the RunPython migration code actually works.

Fixes: #802