Bug 718066 - Enable Firefox Health Report in browser app; r=mconnor

This can't land in mozilla-central until the UX pieces land.
Bug 812608 - Part 1: Persistent storage backend for metrics data; r=rnewman
Bug 812608 - Part 2: Refactor FHR on top of new Metrics APIs; r=rnewman

This also includes a lot of revamped Firefox Health Report features. The
payload format has changed. There is now robust service shutdown logic.
There are more prefs to control behavior. It's almost a rewritten
service.
Bug 826580 - Record application version history in Firefox Health Report; r=rnewman