Refactor Events Backend Based on Frontend Refactor

[ajaygandecha]

This ticket adds a to-do list on some possible improvements to the events backend as I refactor the frontend:

• Ensure that registration data does not emit all user details - should only emit profile name. Then, create a pagination API route for EventRegistration[] rather than just User[], and replace getRegisteredUsersForEvent with getEventRegistrations on the frontend.
• Rename the register for event API to be a post as follows: /api/events/register. The body should be the event ID to register for. Same for unregistering.
• Refactor the events pagination such that by default, filtering by string does not use the time parameters.