Fixed mentions for deleted tickets.

[tipue-dev]

Proposed change

Currently Mentions, that are linked to a ticket, don't get removed when the ticket is deleted. Without a ticket, the Mentions can't be removed anymore by an agent. This change removes all Mentions from a ticket when it gets deleted.

Checklist

• [x] The code change is tested and works locally.(❗)
• [x] There is no commented out code in this PR.(❕)
• [ ] You improved or added new unit tests.(❕)
• [x] Local ZnunyCodePolicy passed.(❕)
• [ ] Local UnitTests / Selenium passed.(❕)
• [x] GitHub workflow CI (UnitTests / Selenium) passed.(❗)

[rkaldung]

Please don't forget a migration module for the cleanup of existing orphans.

[tipue-dev]

Please don't forget a migration module for the cleanup of existing orphans.

This is my first time in that corner of the code. If i should make something differently let me know

[tipue-dev]

Tested the migrations script, it's working now too