Add textarea setting type & refactor some code to clean up

[dsevillamartin]

Refs #2809

Changes proposed in this pull request:

• Refactor some code for buildSettingComponent to reduce duplication
• Add textarea setting type

Reviewers should focus on:

• Code look good?
• Didn't add text editor because that was more complicated

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.