Always try to create users in target wiki

This changes the default behavior to always create and link users on the
local target wiki by using CentralAuth/SUL.

Fallbacks are:

• using a prefixed interwiki user
• if no prefix is defined the unlinked username with the default prefix

The added PHPUnit test overwrites the Hook called in the username handling
to check if this path can be reached and the user will be created.

Bug: T209346
Change-Id: I0d61fb11c03fa540a5f44f451f9cf12cd1b23409