R1:9180f429eb9f

Provide a general-purpose, modular user cache for settings and other similar data

Summary:
Ref T4103. Currently, we issue a `SELECT * FROM user_preferences ... WHERE userPHID = ...` on every page to load the viewer's settings.

There are several other questionable data accesses on every page too, most of which could benefit from improved caching strategies (see T4103#178122).

This query will soon get more expensive, since it may need to load several objects (e.g., the user's settings and their "role…