Move "Preferences" to "Settings"

Summary:
It makes more sense to just make this a settings panel rather than a standalone
app, particularly since setting panels are relatively well separated now.

Also default-disabled the SSH Keys interface since it won't currently be useful
for most installs.

Test Plan: Edited preferences.
Reviewed By: jungejason
Reviewers: tuomaspelkonen, jungejason, aran
CC: aran, jungejason
Differential Revision: 716