Multiple notifications of the same type

[Changaco]

Some notifications, for example the new payment_schedule_modified, can be triggered by user actions. As such, when a user performs multiple similar actions, multiple similar notifications can be generated. Ideally, instead of sending each notification immediately, we would wait a little and compile all notifications ~~of the same type~~ into a single one. Alternatively we could generate the combined notifications in cron jobs instead of when the user performs an action, however this requires keeping enough information in the database (e.g. marking scheduled_payins as "deleted" instead of deleting them immediately).