fix: deleting a discussion from the profile does not visually remove it

[SychO9]

Fixes #2233

Changes proposed in this pull request:

• Based on previous PRs, I think the best approach to this is as Clark suggested. An event-listener approach, so that any discussion list that deletes a discussion, affects all discussion list states.
• This is cleaner than previous PRs.

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.

Required changes:

• Related documentation PR: (Remove if irrelevant)
• Related core extension PRs: (Remove if irrelevant)