Fix mail settings select component never being used

[clarkwinkelmann]

Changes proposed in this pull request:
Fixes a regression introduced in beta 16 with #2593 where the select-style mail settings stopped being rendered as selects. This affects the Mailgun region select which becomes a text input without any help as to what should be entered.

Screenshot
Beta 15:

Flarum 1.1:

After fix:

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.

[askvortsov1]

Tested locally, LGTM. Thanks!