Entity Worker Improvements

[ktuite]

We're going to use a nested transaction inside the entity worker so that we can rely on the database constraints to flag any errors about entity creation. The "current" (before this issue gets fixed) version of the entity worker does some weird stuff like has a complicated pre-check query to test if the entity can successfully be inserted. We'll be able to do away with that extra query with this new approach.

We also need to clearly handle each of the possible submission processing outcomes:

1. it could return null (the submission doesn't create an entity)
2. it could successfully create an entity and return it
3. there could be a validation error
4. there could be a database constraint error
   a. there could be an error that comes from attempting to reprocess a submission that has already been processed, and we could do this with a db constraint or with some other pre-check
5. there could be a validation error that comes from the fact that we're trying to process an encrypted submission (see more Verify that encryption + entities doesn't break things #668)
6. there could be some other kind of internal error, e.g. a database connection problem, that is a kind of error we'd like to have the worker retry