Whens submitting a new object, the instructions for how to do markdown
didn't show up until after the object had been saved once.
This turns out to be because the performance optimization to take an
empty string markdown and turn it into an empty string html wrote the
(empty) result to the wrong field, thereby overwriting the instructions.
* Get rid of the django_markwhat dependency, and implement our own
classes to get more control. In passing also remove django-markdown,
because we never used that.
* Instead of trying to clean markdown with regexps, use the bleach
library (NEW DEPENDENCY) with special whitelisting of allowed tags
based off standard markdown. This means that one can input links or
formatting in HTML if one prefers, as long as it renders to the same
subset of tags that markdown allows.
* Replace javascript based client side preview with an actual call to a
preview URL that renders the exact result using the same function,
since the use of showdown on the client was increasingly starting to
differ from the server, and since that cannot be kept secure the same
way. Rewrite the client side javascript to work better with the now
longer interval between updates of the preview.
Long in planning, but never got around to it.
Suggestion to use bleach for escaping from David Fetter.
