Add footer link to report a bug

Summary:
According to http://www.phabricator.com/docs/phabricator/article/Give_Feedback!_Get_Support!.html, we love feedback.
But there is no way to provide it without reading the docs.

Test Plan: Clicked the link.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin, ldemailly

Differential Revision: https://secure.phabricator.com/D2975