Provide a rough, unstable API for reporting coverage into Diffusion

Summary:
Ref T4994. This stuff works:

  - You can dump a blob of coverage information into `diffusion.updatecoverage`. This wipes existing coverage information and replaces it.
  - It shows up when viewing files.
  - It shows up when viewing commits.

This stuff does not work:

  - When viewing files, the Javascript hover interaction isn't tied in yet.
  - We always show this information, even if you're behind the commit where it was generated.
  - You can't do incremental updates.
  - There's no aggregation at the file (this file has 90% coverage), diff (the changes in this commit are 90% covered), or directory (the code in this directory has 90% coverage) levels yet.
  - This is probably not the final form of the UI, storage, or API, so you should expect occasional changes over time. I've marked the method as "Unstable" for now.

Test Plan:
  - Ran `save_lint.php` to check for collateral damage; it worked fine.
  - Ran `save_lint.php` on a new branch to check creation.
  - Published some fake coverage information.
  - Viewed an affected commit.
  - Viewed an affected file.

{F151915}

{F151916}

Reviewers: chad, btrahan

Reviewed By: btrahan

Subscribers: jhurwitz, epriestley, zeeg

Maniphest Tasks: T5044, T4994

Differential Revision: https://secure.phabricator.com/D9022