[bug 819385] Implement aggregated l10n metrics page.

The new page lives at: http://localhost:8000/en-US/kb/dashboard/metrics/aggregated

Here is a screenshot: http://cl.ly/image/0Q0H3n3u0U2v

If you don't have the google analytics api credentials, the locales picker will sort locales alphabetically and things might not make as much sense or be as useful.

The locale picker needs some serious UX love as it is painful to select different sets of locales. I want to start with this minimal UI and work with the sumo team to figure out how they are going to use it and come up with something easier to use.

r? (@mythmon)