Add sparklines to some of the email reports.

Summary:
They show the last two weeks of data, or at least whatever of those last two
weeks has been saved.  (If we want, we can do a backfill pretty easily, I can
either write a command or we can just rerun the emails from the last 2 weeks.)
Right now, they're all black; I'll work on adding color.

I added them to the instance hours, OOMs, and memory increases reports.  I
think a graph for each RPC type would get to be a bit much, so maybe I'll add
that after we can compute RPC costs.

Test Plan: Redirect the emails to myself by editing `_send_email` to hard-codedly send to me.  Check that things render right, and that if I delete some of the backfilled data I see gaps in the graphs, or sometimes "(insufficient data)".

Reviewers: csilvers

Reviewed By: csilvers

Subscribers: chris, benkomalo, alan, marcia

Differential Revision: http://phabricator.khanacademy.org/D11668