Add locale linter.

To use this do:

    ./scripts/localelinter.py FILENAME

For example:

    ./scripts/localelinter.py locale/ko/LC_MESSAGES/messages.po

It hasn't been exceedingly well tested and I think it occasionally
highlights things it thinks are errors but are "fine" in the sense
that they won't kick up a server error. But, I think this is a
super first pass.
[bug 843237] Add Firefox 21, default to Firefox 19.
[bug 843249] Change Firefox 17 to be the new ESR
[bug 843237] Nix Firefox 21

Verdi only wanted me to default to Firefox 19 and I did too much. Oops.
[bug 783707] Allow disabling of users