Enhance experience of notifying users when registration is successful

[steeevin88]

the way we designed SCEvents is that registration is a POST request to queue the registration to Kafka, which means:

• the POST request basically always returns a success --> registrations should always be enqueued to Kafka (assuming there a spots at the time of registration)
• if the registration is processed by the queue... there isn't actually a "return" to the registrant similar to a traditional REST API --> we dont really work around this, we just redirect users to the events page after registration + the backend processing is quick enough such that if the registration was a success the frontend should say "registered for event" lol

let's figure out a better solution...

• maybe the easiest way is just have the page hang with a "processing" thing until registration is confirmed (right now i believe we redirect to the /events page)
• we can handle hangs w time outs.... maybe render a loading spiner + redirect to homepage after a time out