Add (M) icon for high-memory requests.

Summary:
It goes right next to the (D) for duplicate RPCs.  It is pink for very
high-memory requests (>10MB).

Test Plan:
Load pages, check that the icon appears when it should, and is the right color
on various backgrounds.  Check that it doesn't break other profiling modes.

Reviewers: alan

Reviewed By: alan

Differential Revision: http://phabricator.khanacademy.org/D11243
Make mini profiler detail window float at the top of the page

Summary:
Previously if you had a really long list of AJAX requests in the mini profiler,
you would have to scroll down, click on one, and then scroll way up to the
detail window.  This way, the detail window will float down the page with you.
{F33796}

Test Plan: Load a page, get it to make lots of AJAX requests.

Reviewers: alan

Reviewed By: alan

Differential Revision: http://phabricator.khanacademy.org/D11272