Send balance update mail to all balance import users
Add a button to send a balance update email for balance imports that are manually added. This email should be send both to existing users, and to users that do not have registered an account yet.
- Toggle: send to registered users
- Toggle: send to unregistered users
- Allow adding custom text (remember as part of import event?)
- Toggle: request unregistered users to register
Decide whether to support payment processing through Barbapappa for unauthenticated users.