PLATFORM-75 Remove EmaillerJob

Along with the only extension that references it, EmailUsers, which does
not appear to be loaded on any wikia, and all global variables that are
no longer in use as a result.
Merge branch 'dev' into jobqueue-migration-emaillingjob

Conflicts:
	includes/AutoLoader.php
	includes/DefaultSettings.php
Merge pull request #3918 from Wikia/jobqueue-migration-emaillingjob

PLATFORM-75 Remove EmaillerJob