feat: allow using utf8 characters in tag slugs

[SychO9]

Fixes #3429

Changes proposed in this pull request: Changes the backend endpoints to URL-decode slugs before looking up the tag.

Necessity

• [x] Has the problem that is being solved here been clearly explained?
• [x] If applicable, have various options for solving this problem been considered?
• [x] For core PRs, does this need to be in core, or could it be in an extension?
• [x] Are we willing to maintain this for years / potentially forever?

Confirmed

• [x] Frontend changes: tested on a local Flarum installation.
• [x] Backend changes: tests are green (run composer test).
• [x] Core developer confirmed locally this works as intended.
• [x] Tests have been added, or are not appropriate here.