This connects the FildDiffStore to the HTTP frontend.
Adding support to get image URLs for specific digests and
serve them via the HTTP frontend.

The HTML templates only contain rough layout, no logic.

BUG=skia:

Review URL: https://codereview.chromium.org/671263002
alertserver: Only allow logged-in users @google.com to modify alerts

BUG=skia:3036

Review URL: https://codereview.chromium.org/677773002