Add Event Registration Feature Full-Stack (#212)

Adds functionality to the frontend to allow users to register for/unregister from events.

Major Changes:

* Added a button in the Event Detail Card for users to register/unregister for events.
* Added service methods to the Event Service for getting event registrations/registering/unregistering/etc.
* Created EventMember and EventOrganizer objects to display registered users without exposing all personal information about the users.
* Refactored Event models to store is_registered, is_organizer, organizers, and attendees fields. The latter two fields store EventMember objects to avoid user information exposure.
* Updated frontend to accommodate new backend models and API routes.
* Added an API route to retrieve a paginated list of users registered for an event.
* Added a paginated registrations card to the Event Details page to display users who are registered for an event.
* Added support on the backend and frontend for setting organizers manually (rather than just defaulting to the user creating the event).
* Surfaced the registration limit field onto the frontend and added a form control for setting/updating the registration limit. *Note: Setting the registration limit to 0 when creating/editing an event sets the can_register field to False!

---------

Co-authored-by: Ajay Gandecha <[email protected]>
Co-authored-by: Kris Jordan <[email protected]>
Co-authored-by: Kris Jordan <[email protected]>

backend/api/events/__init__.py

@@ -0,0 +1,15 @@
+from fastapi import Request
+from fastapi.responses import JSONResponse
+from backend.services.exceptions import EventRegistrationException
+
+
+# FastAPI Middleware Exception Handler for ReservationExceptions
+def _event_registration_exception_handler(
+    request: Request, e: EventRegistrationException
+):
+    return JSONResponse(status_code=422, content={"message": str(e)})
+
+
+exception_handlers = [
+    (EventRegistrationException, _event_registration_exception_handler)
+]