CON-2271 fix categories autocomplete on edit page
Remove $wgEnableUserLogin variable and unused code.

This change is one of many cleanups along the way of the Helios project.

`$wgEnableUserLogin` is set to on the config level `true` with no
exceptions. As Wikia's UserLogin extension is now a critical part of
user-related flow in the application, noone is going to remove it.

As a matter of fact, the extension is enabled even for Uncyclopedia and
Wikia One.

Parts of the extension will be refactored as the authentication project
(Helios) progresses.

There is no need to check for `$wgEnableUserLogin` and no need to keep
code that will never be executed or wrap code that will always be
executed.
Merge pull request #5578 from Wikia/dev-platform-625-helios-cleanup-wgEnableUserLoginExt-externaluser

Remove $wgEnableUserLogin variable and unused code.
CON-2273 trigger events for optimizely
Merge pull request #5582 from Wikia/CON-2273

CON-2273 trigger events for optimizely
Merge pull request #5559 from Wikia/CON-2271

CON-2271 fix categories autocomplete on edit page
MAIN-3274 CORS Allow-Origin | Fix refactoring errors
Merge pull request #5584 from Wikia/MAIN-3274

MAIN-3274 CORS Allow-Origin | Fix refactoring errors
ADEN-1478 Sort the keys in adTracker.track data
ADEN-1478 Prepare for new Amazon API
ADEN-1478 New Amazon API
ADEN-1478 Support for both old and new integration
ADEN-1478 Fixing AdTracker tests