WebComment: general comment format improvements

* Introduces the CFG_WEBCOMMENT_ENABLE_HTML_EMAILS config variable.
  When True, emails will also include HTML content.

* Introduces the body_format column for cmtRECORDCOMMENT that describes
  the format of the body of each comment ("HTML", "TEXT", etc.).

* Handles comments appropriately when storing and displaying them,
  according to their body format and desired output format. (closes #2064)

Co-Authored-By: Nikolaos Kasioumis <nikolaos.kasioumis@cern.ch>
Signed-off-by: Nikolaos Kasioumis <nikolaos.kasioumis@cern.ch>