Add a preference to completely disable the file tree

Summary:
See D4812.

  - This preference disables the file tree completely.
  - It defaults off, so users who want it will have to go turn it on.
  - Maybe slightly cleaner would be doing this if the tree was hidden and then ajaxing it in if you press "F", but that's complicated and I don't want to bother.
  - Generally, I think this element is useful to something like 5% of users and not useful to 95%.

Test Plan: Enabled and disabled file tree. Looked at commits and revisions; verified they reflected the setting correctly.

Reviewers: vrana

Reviewed By: vrana

CC: aran

Differential Revision: https://secure.phabricator.com/D4813