allow multiple keywords and formulae in arXiv front end

[kohlhase]

Currently, we can enter multiple keywords in the text slot and one formula in the formula slot. This poses two restrictions:

• we cannot enter key phrases (I think), since their parts are interpreted as different keywords. But I am not sure, see clarify what we can do in the text search bar.  #88
• we cannot enter multiple formulae (this is the more important part of this issue). I think we should allow multiple formula slots (adding one by a + button) with their own previewers.
  We should probably when we have clarified the search semantics (see clarify search semantics #90)