Allow to pass `theorems` in prove-request

There already was a `theorems` command, which was not used in the
RESTful API. It can now be passed to select theorems to be proven. If
it is empty or omitted, all theorems in the node/theory will be proven.
Merge pull request #1428 from eugenk/1425-allow_selection_of_proof_goals_in_restful_api

Allow to pass `theorems` in prove-request