[bug 896477] Add link from l10n dashboard to l10n team.

This was a little more work than I expected because I had to go fiddle with URLs and stuff.

* Added a sidebar nav to go back and forth.
* Added ability to go to l10n dashboard of locales other than the current request locale. To do this I had to fiddle with URLs a little bit.

r?

Test it here, for example: http://localhost:8000/es/localization