chore: use @php in Blade templates

[davwheat]

Changes proposed in this pull request:

As per title.

Reviewers should focus on:

None

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.

[askvortsov1]

Eh, not sure I agree with this. If we're taking action here, we might as well fix the issue: it shouldn't be too challenging.

[davwheat]

I think "issue" is stretching it. As far as I can see, @php is just alternate syntax to <?php, isn't it?