comments: display comment previews while submitting Instead of just saying 'Submitting' and not showing any progress to the user until the comment has been accepted by the server, show a preview of the comment above the comment box in a way which is makes it obvious to the user the comment is being submitted. Apart from that, also clear the comment box so that a repeated clicking the submit button doesn't result in a duplicate comment. The preview doesn't highlight URLs or support @mentions or *bold*, which is a good enough approximation in this case. When/if we (re-)add the rST/Markdown support, we will need a client-side parser for the syntax we choose. When the submission fails, display a message and offer the user to retry or cancel the submission.

# requirements.txt file for use as "pip install -r requirements.txt" as a
# readthedocs compatible alternative to "pip install -e ." which is a working
# alternative to "setup.py develop" which doesn't work with Mercurial 3.7
.