Add a cancel button to Phriction's edit interface

Summary: So you don't have to edit if you don't want to.
Test Plan: Hit "cancel" on an edit of an existing page and an edit of a new
page.
Reviewed By: codeblock
Reviewers: hsb, codeblock, jungejason, tuomaspelkonen, aran
CC: aran, codeblock
Differential Revision: 675