Remove usage of TestGroup (and aggregate)

Summary:
This replaces all uses of TestGroup through the API and frontend with simple TestCase usage.

- Test endpoint is now TestCases
- Project test endpoints now use testcase
- State/controller fixed for project test details
- Add "test report" for showing a paginated version of all tests

Due to the high write load caused by this we're opting to process trees in real-time if the feature is needed in the future.

Reviewers: cf

Reviewed By: cf

Subscribers: zorkian

Differential Revision: https://tails.corp.dropbox.com/D44629