integrate rack-mini-profiler for in-app perf measurements

Only site admins with the "Application Profiling" permission will see
this UI or be able to trigger profilings. The little UI that shows up in
the top left corner contains information on request timings, renderings,
AR queries and timings, and other information about the request.

When Redis is configured, you can share this information with others
(until it drops out of Redis) using a special link given in the UI.

closes CNVS-12563

test plan:

As a full site admin
  * enable the profiling UI by appending ?pp=enable to a request
  * visit canvas pages and verify you can see the new profiling UI

Verify that for all non site admins, the UI doesn't show up and cannot
be enabled with ?pp=enable. It also can't be enabled by other site admin
roles unless permission is given in the site admin permissions UI.

You don't need to do in-depth testing of the UI itself, though do a
quick smoke test to make sure it doesn't interfere with canvas at all or
modify the HTML returned for non-site-admins.

Change-Id: I4e71389af6b56bdff3617170c16b2ee70cea5c6f
Reviewed-on: https://gerrit.instructure.com/33541
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Cody Cutrer <cody@instructure.com>
QA-Review: August Thornton <august@instructure.com>
Product-Review: Brian Palmer <brianp@instructure.com>