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

Ghost User
2018-03-26T16:11:31+02:00

Closes #494

### 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.