Add logging functionality

On request by some presidium members:
Because some people just can't be trusted, it is essential to have some logging functionality on a per event basis. This would also work preventive because people know all their actions are logged.

Possible events:
- first name last name (ugent login) uploads CSV file (link to file?) to change payment states.
- first name last name (ugent login) has changed payment state of ticket X from unpaid (€ 10) to paid.
- first name last name (ugent login) has changed payment state of ticket X from unpaid (€10) to unpaid (€ 1).

Todo list: 
- [ ] Let paper_trail track updates of a registration entry
- [ ] Display these updates in the info modal