fix(tags): Edit tags icon fontawesome url to v5

[dian-kaagman]

Fixes #0000 (no issue)

Changes proposed in this pull request:
The Fontawesome url referred to the newest icons overview, since the tags extension only supports v5 icons it's confusing for admin users why some icons don't work. The url for supported icons is: https://fontawesome.com/v5/icons?m=free.

Reviewers should focus on:

Screenshot

QA

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.