Improve some settings-related performance

Summary:
Ref T4103. Two small improvements:

• Don't work as hard to validate translations. We just need to know if a translation exists, we don't need to count how many strings it has and build the entire menu.
• Allow getUserSetting() to work on any setting without doing all the application/visibility checks. It's OK for code to look at, say, your "Conpherence Notifications" setting even if that application is not installed for you.

Test Plan: Used XHProf and saw 404 page drop from ~60ms to ~40ms locally.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T4103

Differential Revision: https://secure.phabricator.com/D16046