[bug 889928] Locale metrics page + graphs

This creates a new page that is linked off of the KB and L10n dashboards. For example:

http://localhost:8000/kb/dashboard/metrics/es

I moved the existing Helpful Votes graphs from the dashboards and added a chart for % l10n and active contributors.

Most of the stuff is just the boilerplate~ish stuff for graphs, except for some js for splitting and reshaping the data returned by the API.

r? (@mythmon)