Move RevisionState on WorkerVersion

Bad design decision 😿 : the state must be on the WorkerVersion instead of the Revision:

1. remove state field from Revision
2. add a state field with the same values as RevisionState (which is renamed to WorkerVersionState)
3. allow retrieving & updating worker revision state field through the API
4. add a computed property state back on Revision which behaves like this:

• if there is a worker version in error, use error
• elif there is a worker version in processing, use processing
• elif all worker versions are available, use available
• else (no worker version or other states) default to created