fix: use app baseUrl instead of location.origin

[davwheat]

Changes proposed in this pull request:

PostMeta contains a permalink to a post, however this link is generated with window.location.origin instead of app.forum.attribute('baseUrl'), meaning that if Flarum is installed as a subdirectory, these links would be invalid as they would miss the subdirectory.

Reviewers should focus on:

N/A

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.