Demolish repository processes

https://redmine.teklia.com/issues/5720

• Remove ProcessMode.Repository

• Create a data migration that:

  1. creates a new corpus named Repository processes, with a description explaining that old Git import processes are migrated here
  2. updates the mode of Repository processes to Workers, and the corpus ID to this new corpus

  If there are no Repository processes at all in the database, then it shouldn't do anything.

  This corpus is necessary since other process modes require a corpus to be set. We cannot just delete old repository processes, since they provide artifacts that may still be in use in some WorkerVersions. There are a few thousand versions relying on it right now on each of our instances.

• Remove the Process.revision_id column

• Remove revision_id from CreateWorkerVersion, since it was only used by the tasks removed in tasks#188 (closed)

• Remove support for the revision ACL checks in the ACL mixins