Make Diffusion view sticky

Summary: Blame can be slow.

Test Plan:
Viewed a file with no preference, saw blame.
Changed view, saw it.
Viewed a file, saw the changed view.
Viewed a file as raw document.

Reviewers: Two9A, epriestley

Reviewed By: epriestley

CC: aran, epriestley, Korvin, btrahan

Maniphest Tasks: T1278

Differential Revision: https://secure.phabricator.com/D3000