[admin] include an optional step to create an ID via Endpoint

[dariok]

• configuration setting which endpoint to use or whether this is active
• and/or a generic endpoint that usually returns 204 but for which a function can be implemented if it returns 200 and a body, use that as the ID for the project

Sth. similar should be possible for files: a form to create a (largely empty) TEI file with some meta data for which an ID is automatically created

NB that for multi-server setups only one endpoint in one instance should be used to avoid problems.

[dariok]

While this is still interesting – automatically generating IDs for projects and/or files will be helpful if it can be made stable, i.e. avoiding creating several IDs for one file if it is re-uploaded –, this will have to wait until after REST v2 is stable (but probably before dropping v1)

Cf. #574