Steal WorkerActivities from other processes in initialize_activity

Closes #1357 (closed)

While removing the activity-stealing abilities of UpdateWorkerActivity, I noticed that no tests failed at all, so I first fixed a test so that it broke, and then updated the test again.

I could not use Django's conflict handling at all simply because we do not use bulk_create at all, as we need an INSERT … SELECT and not a regular bulk create. The ON CONFLICT clause I had to use could never have worked with Django anyway as I needed an index predicate to match the relevant unique constraint, and a WHERE on the update itself to prevent stealing activities that were processed or started, as that would defeat the point of worker activities (don't run on what you already processed!).