Support required types for workers

A worker should be able to require certain element types to be present:

• to produce transcriptions, a worker will need text_line elements
• to detect faces, a worker will need face elements
• to detect enluminures, a worker will need such a type

The idea is to declare those types in the worker configuration, store it in the database, and check that a corpus has those types when creating a new worker run.

This will require the development of an element type management interface #337 (closed)

Need to discuss/approve this with @kermorvant