Introduced $wgULSAnonCanChangeLanguage

This allows us to enable web fonts and input methods for all
users by disabling just the interface language selection.

Updated the documentation of the variables to make it clearer
what each does. Summary:
* Enabling: $wgULSEnable and $wgULSEnableAnon
* Workarounds: $wgULSAnonCanChangeLanguage and $wgULSLanguageDetection

Cleaned up hooks, in particular rewritten getLanguage to separate
the logged in vs. logged out steps for easier understanding.

A followup commit is needed to disable language selection UI for
anon if $wgULSAnonCanChangeLanguage is false.

Change-Id: Ia8d21c394ff5efac0ce94664710c97dc3b74ec18
Merge "Introduced $wgULSAnonCanChangeLanguage"