Adds cost numbers to email reports.

Summary: Now that we have them, might as well use them!  Sparklines to follow.

Test Plan: Sent the email to myself, it worked.

Reviewers: csilvers

Reviewed By: csilvers

Subscribers: alan, benkomalo, chris, marcia

Differential Revision: http://phabricator.khanacademy.org/D11913
Add sparklines to RPCs email for cost/req.

Test Plan:
Sent the email to myself, it worked (with missing old data, and with old data
missing the 'rpc_cost' field).

Reviewers: csilvers

Reviewed By: csilvers

Subscribers: benkomalo, marcia, alan, chris

Differential Revision: http://phabricator.khanacademy.org/D11916