[2.x] feat: select own post permission

[imorland]

Fixes #0000

Changes proposed in this pull request:

Removes the previous Select own post as best answer setting and replaces it with tag scoped permissions, to allow for finer control over which users can perform this action

Reviewers should focus on:
Note the translation key change from fof-best-answer.admin.settings.allow_select_own_post to fof-best-answer.admin.permissions.allow_select_own_post

Screenshot

Confirmed

• Frontend changes: tested on a local Flarum installation.
• Backend changes: tests are green (run composer test).

Required changes:

• Related Flarum core extension PR's: (Omit this section if irrelevant)

[DavideIadeluca]

As you mentioned I think we need a migration here (especially considering the pending 1.x implementation) which:

• If Select own post disabled, nothing needs to be done (admin by default, fully preserves backwards compatibility)
• If Select own post enabled, grant member permissions globally and optionally on all tag scopes (should also be fully backwards compatible)

Also it appears like a change in the condition of adding the frontend controls are needed as currently the controls are added whilst the backend returns a 403 response:

Screen.Recording.2025-12-02.at.09.40.04.mov

[imorland]

Thanks for testing @DavideIadeluca - I've not added the migration yet, but the permission issue you identified is now resolved locally for me. Can you please confirm?

[DavideIadeluca]

The previous issue is now resolved, but now it appears like a new issue is introduced with unselecting a best answer (with permitted users):

Screen.Recording.2025-12-02.at.13.06.46.mov

[DavideIadeluca]

tested locally, lgtm