When a page is profiled, profile all AJAX requests too

Summary: If a page is profiled, add an "X-Phabricator-Profiler" header to all Ajax requests, and profile those too.

Test Plan: Profiled a page, checked Darkconsole, saw profiles for everything.

Reviewers: vrana, btrahan

Reviewed By: vrana

CC: aran

Differential Revision: https://secure.phabricator.com/D4885