[DO NOT MERGE] - Helios Implementation, version 1 (when rounded up)

## Important ##
* **Commits that require a thorough review and understanding are marked with `[CR]`.**
* **Corresponding changes in config: https://github.com/Wikia/config/pull/752.**

## Changes ##
* `wfGetSessionKey()` that calculates memcached key for current session data has been simplified.
* `$wgExternalUserEnabled` global variable (`true` always) has been removed from code and config.
* `HeliosExternalUser` class has been introduced and will be replacing `ExternalUser_Wikia` as the development progresses; it is included right after `ExternalUser_Wikia` in `CommonExtensions.php`.
I have checked it before - see the `extensions/wikia/Helios/README.md` :-) But thanks, anyway! Good catch!