[bug 793850] Add product/version filters to dashboard

This isn't super pretty, but it's functional.

1. shows products as faceted navigation based on "browser" field in Response model
2. when you choose a product, then it shows versions for that product as faceted navigation
3. both browser and browser_version are in the dashboard atom feed urls

I also fixed the tests for the atom urls so they're less order dependent, but the code looks gross.

r?