Add a note to the .rich-text styles about email_markdown.py

I meant to add this to my previous change which added markdown rendering for
HTML, but I forgot to commit it before pushing, so it just sat in my working
dir. Trivial change.
Update locale files.

Just a simple run of makemessages, in preparation for uploading to transifex.
Change MarkdownEditor to allow tabbing between form elements.

CodeMirror, by default, handles tabs specially, as it's intended for
editing source code. We want them to behave closer to textareas, so we
want the Tab key to switch focus instead. This turns off CodeMirror's
tab support.