Rich text rendering in Hyperkitty

Adding support for Rich text rendering in Hyperkitty is a little tricky. The idea being that we pre-process part of the POST to quote which doesn't play all too well with directly putting the post through markdown renderer.

Rich text in this context refers mostly to Markdown but I would also love if we could support basic ReST since we are python peeps :) readme_renderer that we have been using in Postorius supports both and is used by PyPI(warehouse) so I would assume is safe to render both formats.

It would be really great if we could just support some of the HTML elements, if not all, the top one on my list is <code> or in markdown world triple quote backticks.

def python():
  pass

Something like this.

The 2nd thing would be to allow embeded images in the posts. This needs to be implemented carefully and we don't want to pull in any remote images but only the ones attached. This should also be configurable per-mailing list and should be gated through a side-wide configuration setting for administrators who don't care about this at all.