[CR] Use HeliosExternalUser rather than ExternalUser_Wikia.

The `HeliosExternalUser` class extends `ExternalUser_Wikia` and
at this moment does not introduce any new logic so this change
is formal.
Merge branch 'helios-sanitize-authplugin-and-externaluser' of github.com:Wikia/app into helios-sanitize-authplugin-and-externaluser