Move form field partial to a component

This way it's easier to test; changing it will also be easier.

During my experiments I made a mistake which wasn't covered by the test
suite. We're adding a test for this case.

Note we're using `i18n_content` in the component instead of `content`
because there's already a `content` method provided by ViewComponent.

spec/components/admin/site_customization/information_texts/form_field_component_spec.rb

@@ -0,0 +1,14 @@
+require "rails_helper"
+
+describe Admin::SiteCustomization::InformationTexts::FormFieldComponent do
+  after { I18n.backend.reload! }
+
+  it "uses the I18n translation when the record exists without a database translation" do
+    I18n.backend.store_translations(:en, { testing: "It works!" })
+    content = I18nContent.create!(key: "testing")
+
+    render_inline Admin::SiteCustomization::InformationTexts::FormFieldComponent.new(content, locale: :en)
+
+    expect(page).to have_field with: "It works!"
+  end
+end