Add user preference to enable ULS

Adds an option, 'enable-uls' for users to enable ULS. By default
it is enabled. Wikis can use $wgHiddenPrefs to prevent users from
disabling ULS.

Bug: 46306
Change-Id: I71b70d8ee7c3cad7f49b32e5dc494ef4fc1bdb2f
(cherry picked from commit ad9caf32be4326f48e79e10edff9eddac74d34e3)