don't set "follow by email" as default when opening a new thread

What does this MR do?

This MR changes the default follow behavoir when opening a new thread. Afterwards only bells will be generated, but no emails anymore.

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

Tests to weird things locally, so lets see, if they do that in the CI too.

Links to related issues

There was a request in the product team: https://foodsharing.de/?page=bezirk&bid=2296&sub=forum&tid=114121 with a vote

Checklist

• added a test, or explain why one is not needed/possible...
• no unrelated changes
• asked someone for a code review
• set a "for:" label to indicate who will be affected by this change
• use "state:" labels to track this MR's state until it was beta tested
• added an entry to CHANGELOG.md
• add a short text that can be used in the release notes
• Once your MR has been merged, you are responsible to create a testing issue in Beta Testing Repo:
  • Consider writing a detailed description in German.
  • Describe in a few sentences, what should be tested from a user perspective.
  • Also mention different settings (e.g. different browsers, roles, ...). how this change can be tested.
  • Be aware, that also non technical people should understand.

Release notes text

Beim Anlegen neuer Forenthemen ist die voreingestellte Benachrichtigung nun ausschließlich per Glocke, statt wie vorher Glocke und E-Mail. Natürlich kann bei Bedarf jederzeit auch die Mail-Option aktiviert werden.