Add Local process mode and fake_worker_run command

Closes #1421 (closed)

The database constraints caused some headaches: Some old Repository processes still had corpus linked to them, due to them being old IIIF Git imports, so I added a data migration. 1 (one) Workers process had no corpus defined in preprod, and it got cleaned up due to being expired, so I manually deleted it.

Local processes having neither a corpus nor a revision also mean that the ACL becomes inconsistent, as they can be retrieved, but cannot be updated or deleted like the others, and errors can cause a HTTP 403 instead of 404. This took a while to deal with in unit tests. This will need to be discussed in a larger rationalization of the process access control, maybe for #1172.

Also note that excluding a process mode by default sounds awfully similar to the past handling of top_level in element lists, which we removed in #831 (closed) when implementing the FilterBar to make the implicit behavior explicit.