Enhancement: Edition module

Description

The TextAPI offers (and sometimes requires) users to provide certain metadata that are applicable to every kind of texts.

Since the TextAPI has been designed for usage in the SUB in the first place, we might want to consider what kind of texts we typically deal with. One type are (editions of) manuscripts.

Maybe it's worth a thought if we could provide a set of optional metadata fields that are typically relevant for manuscripts. This way, projects could simply implement the "editions'/manuscripts' module" of the TextAPI if they provide the endpoints for manuscripts.

This would lead to:

1. higher interoperability of the data provided at the endpoints (e.g. for providing information about the date of creation, one project wouldn't come up with x-date-of-creation and the other one with x-creation-date
2. projects not having to think that much about relevant metadata because a core set is already provided
3. the QViewer being able to support these metadata out of the box if we decide to tackle this approach
4. (as a result of 3) projects having to customize the QViewer less if they want to have manuscript/edition specific metadata displayed.

/cc Mathias Göbel, Frank Schneider, Michelle Weidling