Do not show unsubscribed email subscriptions for forum threads in notification settings

Closes #893 (closed)

What does this MR do?

With changes in !1191 (merged) email and bell subscriptions are handled differently by now. In settings there is a menu point "notifications" (=Benachrichtigungen). This allowed to disable a description in the past – which is still working. With current changes the disabled entry isn't removed anymore. Previously it was removed after saving.

This MR ensures that only activated mail notifications are displayed.

How confident are you it won't break things if deployed?

(be honest!)

Links to related issues

Any relevant links (issues, documentation, slack discussions).

How to test

Steps a reviewer can take to verify that this MR does what it says it does e.g.

1. Checkout branch locally
2. Login as foodsaver
3. Disable mail notifications on http://localhost:18080/?page=settings&sub=info
4. Disable and activate mail notifications on the forum threads
5. Check whether the entries in the settings are available or not as desired.

Screenshots (if applicable)

Any relevant screenshots if this is a design / frontend change

Checklist

• added a test, or explain why one is not needed/possible...
• no unrelated changes
• asked someone for a code review
• joined #foodsharing-beta channel at https://slackin.yunity.org
• added an entry to CHANGELOG.md (description, merge request link, username(s))
• Once your MR has been merged, you are responsible to update the #foodsharing-beta Slack channel about what has been changed here. They will test your work in different browsers, roles or other settings