WIP: Resolve "Sorting registrations in events also by name"

Closes #494 (closed)

Short description

Previously, registrations on an event's admin page were sorted only by registration date. Now, the user can sort by Name, Date, Presence or Payment Status.

Steps to use this feature

1. Open an event's registration page.
2. Click or tap a column header for the column you want to sort by.
3. The page reloads with the newly sorted list.