[1.x] [extensibility] feat: allow modifying the discussion title on PostsUserPage

[imorland]

Changes proposed in this pull request:
As title says, extract the discussion title from the view and place it into a function, so that it can be updated from 3rd party extensions

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.