Cleanup: Delete top-level favicon.ico.

It was probably left behind from the days of the Buildbot code.

BUG=None
R=borenet@google.com

Review URL: https://codereview.chromium.org/792713002
Overhaul buildbot package

- Organizational changes:
  A. The database is now considered to be the primary data source. Everything
     related to the build master's JSON interface is now moved into ingest.go.
  B. The buildbot package itself handles database interactions. This makes more
     organizational sense to me, since the module itself should know best about
     how to store builds in the database.
- Adds tests for ingestion and database operations.
- The major missing piece is matching each build to a set of commit hashes,
  which is next on my list.

BUG=skia:3125

Review URL: https://codereview.chromium.org/761433005