SuperSize: Fetch milestones and pre-generate data files

Adds `generate_milestone_report.py`, a script to generate milestone
reports then upload the results to GCS. A new page for selecting a
milestone was also created, which directs the user to Super Size with
a certain data_url set.

Bug: 847599
Change-Id: I718e14814b021c7810e94a0b95bf55e1ed381be6
Reviewed-on: https://chromium-review.googlesource.com/1150738
Commit-Queue: Tiger Oakes <tigero@google.com>
Reviewed-by: Peter Wen <wnwen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#578352}
9 files changed