Auth for Google Analytics using a service account

Summary:
Using a service account means we keep a secret private key file, and
then the code can access Google Analytics without requiring a user or
the one who starts the webserver to auth against Google Analytics' API.

Test Plan:
I copied the <HASH>-privatekey.p12 file from Secrets/ to
analytics/webapps/dashboards/ga_client_privatekey.p12

Then I ran ./main.py -d and loaded http://localhost:5000/is-ka-fast-yet
to verify that the graph appeared and looked right. It did!

Reviewers: csilvers

Reviewed By: csilvers

Differential Revision: http://phabricator.khanacademy.org/D6572