Export registrations to csv

One-sentence description

Export of registrations to csv (the registrations of members for Thalia)

Motivation

To check Thalia's bookkeeping with the Thalia administration it would be nice to export the registrations.

Desired functionality

The csv should contain at least: name of the person, type of membership/registration, amount, date of payment, payment method.
This way it is possible to compare the cash register or bank transactions to the registrations at the website. This is also related to a third payment option (#813 (closed)).

Suggested implementation

Just as for the users (where one can download .csv's), a button 'download registrations'.