fix(notifications): Rework the notification handling

[nickvergessen]

• Don't create notifications for users that signed any terms already
• Don't create notifications when enabling the app, but only after terms have been added
• Don't generate notifications live on resetAllSignatures but use a cron job

Fix #1000

Scenario №1

1. Set up ToS and accept them
2. Close your browser (to not pull notifications)
3. Disable and enable the app 5 times in a row
4. Check the database to see that you received 5 notifications and also received 5 push notifications when a mobile app was set up, even though you accepted them already

Scenario №2

1. Connect a mobile phone to receive pushes
2. Enable the app (but don't configure any ToS)
3. Check the database to see that you received s notification and also received s push notification, even though there are no terms that can be accepted

Scenario №3

1. Have an instance with > 20k users also having mobile phones connected
2. Set up ToS
3. Press "Reset all signatures"
4. Wait for the request to time out as we try to generate 20k notifications and push them

[nickvergessen]

/backport to stable27

[GretaD]

I only tested scenario 1

[backportbot]

The backport to stable27 failed. Please do this backport manually.

# Switch to the target branch and update it
git checkout stable27
git pull origin stable27

# Create the new backport branch
git checkout -b backport/1009/stable27

# Cherry pick the change from the commit sha1 of the change against the default branch
# This might cause conflicts, resolve them
git cherry-pick 03222a56

# Push the cherry pick commit to the remote repository and open a pull request
git push origin backport/1009/stable27

Error: No changes found in backport branch

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Learn more about backports at https://docs.nextcloud.com/server/stable/go.php?to=developer-backports.