Make it possible to claim a task multiple times

[nikslor]

Some tasks can be made generic and it should be possible for multiple users to claim them.

An example: The "LibreOffice" projects adds a task which is about installing an unstable build of the software and users can try to find bugs in it. This task can be claimed by multiple users and mentors should still see which users claimed it.

Maybe we should not call it "claim" but more "I'll work on it"?

A mentor should be able to flag a task as "single claim" or "multi claim" (or similar).