Add an option to lock/unlock editing of the document

[Dagefoerde]

Together with the HS Hannover and Synergy Learning, we are planning to contribute the following enhancement:

A teacher can "lock" (and unlock) a document. When a document is locked, it can only be edited by teachers and it is in read-only mode for students. Otherwise,

(This is one of the tasks resulting from the general idea that I presented in #5.)

[logicexpertise]

Looking forward to your PRs for this proposed feature