Fixes #0000

Changes proposed in this pull request:
Fixing an issue causing users unable to rename their posts, which has been confirmed at #3692

Reviewers should focus on:
This is a rather brutal fix, since I'm not too into flarum. Please consider rewriting this in an elegant way.
At least I have confirmed it working on my own deployment at https://forum.monika.love

Screenshot

Necessity

• Has the problem that is being solved here been clearly explained?
• If applicable, have various options for solving this problem been considered?
• For core PRs, does this need to be in core, or could it be in an extension?
• Are we willing to maintain this for years / potentially forever?

Confirmed

• Frontend changes: tested on a local Flarum installation.
• Backend changes: tests are green (run composer test).
• Core developer confirmed locally this works as intended.
• Tests have been added, or are not appropriate here.