Improve UI of admin site settings (#4163)

app/controllers/admin/settings_controller.rb

app/javascript/styles/forms.scss

app/models/form/admin_settings.rb

app/views/admin/settings/edit.html.haml

config/initializers/simple_form.rb

config/locales/ca.yml

config/locales/de.yml

config/locales/en.yml

config/locales/fa.yml

config/locales/fr.yml

config/locales/he.yml

config/locales/id.yml

config/locales/io.yml

config/locales/ja.yml

config/locales/ko.yml

config/locales/nl.yml

config/locales/no.yml

config/locales/oc.yml

config/locales/pl.yml

config/locales/pt-BR.yml