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>
WebComment: Add & amend tests, fix SQL statement

* Adds and amends unit tests.

* Fixes the SQL create statement for cmtRECORDCOMMENT.

* Improves the upgrade script.
Merge pull request #5 from makistsantekidis/master-webcomment-format-improvements

WebComment: Add & amend tests, fix SQL statement
Merge branch 'master-webcomment-format-improvements' of https://github.com/kasioumis/invenio into kasioumis-master-webcomment-format-improvements
Merge branch 'kasioumis-master-webcomment-format-improvements' into prod