tags: tag individual papers rathers than paper-questions?

The following discussion from !1128 (merged) should be addressed:

• @arechnitzer started a discussion: (+1 comment)

  Perhaps we do want tags to be associated with the paper (ie the QGroup in the DB) rather than the annotation.

See other stuff in the MR about database.

It can stay a big block of text or could perhaps be reworked to some kind of tags table (if that makes SQL queries easier? Find me all papers tagged Colin marked by Andrew between 1am and 6am.)