fix: default labels with SettingDropdown and SelectDropdown

[dsevillamartin]

Changes proposed in this pull request:

• I've removed the array check that was reverted to in fix: empty string displayed as SelectDropdown title #3773. The code if (Array.isArray(label)) label = label[0]; is from 2015. I don't think either that or extractText is needed here -- I tested this in v1 first with no issues in the same examples provided in that PR.

  https://github.com/flarum/framework/blame/f7dd609b264d4266151efa3b8a4c5775526346fc/framework/core/js/src/common/components/SelectDropdown.js#L48
  

• Made the defaultLabel not override custom ones set by vnode attrs. This fixes the fallbacks in PermissionGrid for eg. discussion & tag renaming. They're currently broken in v1 as well.

  The previous change was needed because otherwise it'd only show "For" (since trans returns an array). Mithril can handle arrays just fine - the label[0] looks to me to be from when Mithril perhaps would join with commas (I vaguely remember that?)

• Also translated the "Custom" fallback. Wasn't sure what key to give the translation, since SettingDropdown is only used in PermissionGrid (but technically could be used anywhere in admin, I believe).

Screenshot

Before	After

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.