Use class attribute to represent 'user' and 'group'

When sending a notification a resource_type is required, such as a user
or project. This value is always consistent between the different
notifications.created, updated, deleted methods, so make it a class
attribute.

Change-Id: I084fedb1d8f0c1b1d7db575860d96ff2e5a27647
Merge "Use class attribute to represent 'user' and 'group'"