When the profiler is active, keep it active if the user submits forms

Summary: Ref T12297. When a page is generated with the profiler active, keep it active by adding a __profile__ input to any forms we generate.

Test Plan: Hit Conduit API page with __profile__ active, saw it reflected in forms.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12297

Differential Revision: https://secure.phabricator.com/D17399