chore: avoid using .fa() mixins and @fa-var vars

[SychO9]

Changes proposed in this pull request:

• These were a problem in 1.x when we attempted to upgrade to fontawesome 6, they also pose an issue with regards to possibly moving away from using LESS because of the compiler.
• It's also generally easier to have explicit icon elements and switch between them.

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.