allow user to localize her bio (bug 933038)

This PR adds a "localize for" menu, like the one in devhub, to allow a user to
localize her bio.

The current language is used to select the bio translation to display. If it
doesn't exist, a fallback language is used (by default, the en-US, set in the
settings).

As the user couldn't localize her bio previously, and if she entered her bio
while browsing the site using the "fr" locale, her bio would only be displayed
for users also browsing using the same locale. For anybody else, no bio would
be displayed.

The user can now localize her bio for various locales, including the default
(fallback) one: en-US.

I've also taken the opportunity to revert a change added in
https://github.com/mozilla/zamboni/pull/1708/files#diff-dff2aa5261ee9882eb4ef3405c2e934fR254
(for no particular reason), reverting the limitation to 200 chars max on the bio.