OR-801 Markdown parser can be enabled per-message

* Add `parsers` field for the `base-message` model.
* Update documentation.
* Update helper called by the webserver controller.
* Add a test.
Merge pull request #589 in OR/rse from ~RPIGNOLET/rse:OR-801 to master

* commit '7d067cc1e3e4fffec68f3ebda1c70ce931b75d10':
  OR-801 Markdown parser can be enabled per-message