Notifications Email: Turn off notifications / change frequency

[FinnWoelm]

In the notifications email, I want

• a link to turn off all notifications (login required)
• a link to change my notification frequency (login required)

[FinnWoelm]

In the first iteration, let's make both these a link to the account settings page. That will require a login and may cause frustration on the user side.

Later on, we should create an option for immediate opt-out. Ideally, it should use a secure token and lapse after one week and then also email the user that they were opted out of email notifications. This is to avoid harmful action when a notification email is forwarded to a friend. See #296

[FinnWoelm]

Removed from Sprint because we don't have any teams using Openly and thus the current value of this feature is low.