Prepare ULS for EventLogging

This change introduces a new configuration variable, $wgULSEventLogging. When
set to true, ULS will register an EventLogging schema module for
<http://meta.wikimedia.org/wiki/Schema:UniversalLanguageSelector>,
making it possible to log events using `mw.uls.logEvent`. When false, the
schema module is not loaded, and `mw.uls.logEvent` is a no-op.

Change-Id: I7139e67cc2f830a6b4b3c8ff1fc72dfcdec9a699
Merge "Prepare ULS for EventLogging"