[1.x] chore(core, tags): resolve a11y warnings in Admin Frontend

[DavideIadeluca]

Progresses #3360
The Permission Scope Removal -and Edit Tag Buttons don't have content or an a11y label, which resulted in a bunch of warnings being logged in the console on those pages, which is a minor nuisance while developing/debugging the admin frontend.

Changes proposed in this pull request:
This PR adds the aria-label attribute to two different Buttons, which were the cause for the Flarum Accessibility Warnings and improves QOL for extension developers.

Reviewers should focus on:

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.

Required changes:

• Related documentation PR: (Remove if irrelevant)