escape % characters in strings

The problem here is that perhaps sometimes the character IS actually
for formatting - a db value used later for formatting variables into
a string. Also, this means the translator will likely put %% as the
translation, and because this is not really python, it will
actually show '%%' in the translated data, so the %% need to be
translated to %...

A proper fix would have to detect this properly, and/or format things
back on input, or instead of making it appear to be a python file, use
an html intermediary format.
Vinaigrette adds double-percentages to msgids before looking in the database.

To match how the rest of django and xgettext handle string escaping, all the regular percentages that are not string interpolation expressions must be escaped, even in string that do no need it. Poor xgettext, maybe we are asking too much of it.
Automated merge with https://ecometrica.kilnhg.com/Code/Public/Group/django-vinaigrette