Notification system

[dpausp]

As a user I may want to get notifications for various events, for example:

• someone wrote an argument for/against my proposition
• someone supported my proposition (may be noisy)
• my proposition changed state
• admins have feedback about my proposition
• voting phase has started/will start in n days

We can use ekklesia-notify for that but we have to ask for contact addresses separately in the portal and store them. That differs from a voting component which only sends out notifications for currently logged-in users. For logged-in users, the encrypted recipient info from the OIDC provider can be used but we also want to send notifications to users that are offline.