Move configuration dependent on other extensions into callback

Adds a CentralAuthHooks::onRunExtensionFunctions() callback to
$wgExtensionFunctions. Moves one existing callback and one config block
that should have been made a callback previously to the new method.

Change-Id: Iacbe6a6c8824c9682e8b346d5d8db967f15dbbe7
Merge "Move configuration dependent on other extensions into callback"