FEATURE: Implement new required options in admin user fields UI (#27079)

We're planning to implement a feature that allows adding required fields for existing users. This PR does some preparatory refactoring to make that possible. There should be no changes to existing behaviour. Just a small update to the admin UI.

spec/system/admin_user_fields_spec.rb

@@ -0,0 +1,28 @@
+# frozen_string_literal: true
+
+describe "Admin User Fields", type: :system, js: true do
+  fab!(:current_user) { Fabricate(:admin) }
+
+  before { sign_in(current_user) }
+
+  let(:user_fields_page) { PageObjects::Pages::AdminUserFields.new }
+
+  it "correctly saves user fields" do
+    user_fields_page.visit
+    user_fields_page.add_field(name: "Occupation", description: "What you do for work")
+
+    expect(user_fields_page).to have_user_field("Occupation")
+
+    user_fields_page.refresh
+
+    expect(user_fields_page).to have_user_field("Occupation")
+  end
+
+  it "displays an error when missing required fields" do
+    user_fields_page.visit
+
+    user_fields_page.add_field(name: "Occupation", description: "")
+
+    expect(user_fields_page).to have_text(/Description can't be blank/)
+  end
+end