Dashboard for homepage DOMContentLoaded timings

Summary:
This changeset adds support for a new dashboard to show whether the
homepage is meeting our 500ms performance target yet. A later commit
will do AJAX auto-refresh and an even later commit will do a metric for
out-of-memory issues on the coach reports page.

While I was in the code I improved some documentation.

I created the new "Client ID for native application" credential in the
KA cloud project to support this dashboard.

Test Plan:
I followed the directions in README to set up a virtualenv and then ran
analytics/webapps/dashboards/main.py -d

I then got the "Client ID for native application" credentials at
https://cloud.google.com/console/project/apps~khan-academy/apiui/credential
and put them in analytics/webapps/dashboards/ga_client_secrets.json

I loaded http://localhost:5000/is-ka-fast-yet and authorized the app to
access my Google Analytics data. Then I saw the graph!

Reviewers: csilvers

Reviewed By: csilvers

CC: alan

Differential Revision: http://phabricator.khanacademy.org/D6521