[bug 907748] Add questions metrics to metrics page.

A few things.

* Removed xx locale from AAQ_LANGUAGES
* Repurposed `/questions/stats` into a "metrics dashboard" and moved it to `/questions/dashboard/metrics(/<locale_code>)` for consistency with kb/l10n versions of these
* Added a locale switcher
* Added the same graph that we show on the KPI dashboard but now you can look at it on a per locale basis. I had to tweak the API view to handle a locale parameter
* The existing topic graph now respects the selected locale

That should be it. r?