- Standardize signatures and variable names for extenders that take callbacks
- Adjust model extender docblock to clarify that default calue can't be an invokable class.
- Make invokable classes provided to Model->relationship
- Add integration tests to ensure Model->relationship and User->groupProcessor extenders accept callbacks
- Extract code for wrapping callbacks into central util
* Deprecate GetDisplayName event
* Add interface for display name driver
* Add username driver as default
* Add code to register supported drivers / used driver as singletons
* Configured User class to use new driver-based system for display names
* Add extender for adding display name driver
* Add integration test for user display name driver
* Add frontend UI for selecting display name driver
