SpecialTrackingCategories: Read from the extension registry

This demonstrates how we can transition from extensions putting
things into the global scope ($wgTrackingCategories) to instead
storing them in the extension registry. This will increase the
overall performance of the extension registry since it no
longer needs to do an array_merge with $wgTrackingCategories.

For extensions already converted to using the registry
no change is needed as the schema is still the same.

Change-Id: Ie0df4c20b123dac784a1c02eb991edc609a911b6