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.

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