WebComment: bibdoc relations & filtering

* Creates database table to store the comment-bibdoc relations and
  provides functions to get/set the relations.

* Amends the interface so that users can: (a) relate comments to bibdocs
  when adding new comments, (b) filter the displayed comments based on
  those relations and (c) filter the displayed comments based on free text.

Co-Authored-By: Harris Tzovanakis <me@drjova.com>
Co-Authored-By: Esteban J. G. Gabancho <esteban.gabancho@gmail.com>
Signed-off-by: Nikolaos Kasioumis <nikolaos.kasioumis@cern.ch>