Add test grouping to project test list

Summary:
This re-adds similar behavior to what TestGroup used to provide, computed on demand.

- The tests tab now shows test groups in a folder like structure
- A new "all tests" page now exists (which will eventually become full on search)

Reviewers: ziga, cf

Reviewed By: cf

Differential Revision: https://tails.corp.dropbox.com/D45448