Detect HTML in markdown submission forms

Detect HTML in markdown submission forms

This will do simple html detection based on trivial regexps. If HTML is found,
it will replace it with HTML_REMOVED and also show a warning (in red color,
no less) that this is not permitted.

The actual filtering is still done server-side later, so the fact that these
regexps are trickable if you really want to doesn't matter...

Fixes #164