R1:af1f57b37ac3

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…