Add User Preferences endpoint.

In order to start saving our user preferences to the server (To
support email preferences, for example), this patch adds a new
endpoint under /users/ID/preferences to manage a user's specific
settings. Supported types are float, int, boolean, and string.

Also added some coverage for user browse. Search could not be
covered since our build nodes only run mysql 5.5, and storyboard
requires 5.6.

Change-Id: I51e00d9dedcb392db906efd06745eab56c758261
Merge "Add User Preferences endpoint."