When initializing an extension, we set extension_cls.instance to the

initialized extension, but we do this after some extension-supplied code

(the administration site setup) is run. This causes such code to fail

with either instance being an invalid attribute or being None,

depending on any prior initialization.
We now set this alongside storing the extension instance in the manager,

ensuring that there's always consistency between looking up an extension

via the extension manager and via the instance attribute.

Unit tests pass.