FEATURE: Add a checkbox for users to confirm before flagging as illegal (#25762)

The Digital Services Act requires a checkbox for any user who's flagging a post as illegal to confirm that they are flagging in good faith. This PR adds that.

spec/system/flagging_post_spec.rb

@@ -32,4 +32,22 @@ describe "Flagging post", type: :system do
       expect(page).to have_css(".reviewable-meta-data .status .approved")
     end
   end
+
+  describe "As Illegal" do
+    it do
+      topic_page.visit_topic(post_to_flag.topic)
+      topic_page.expand_post_actions(post_to_flag)
+      topic_page.click_post_action_button(post_to_flag, :flag)
+      flag_modal.choose_type(:illegal)
+
+      expect(flag_modal).to have_css(".flag-confirmation")
+
+      flag_modal.fill_message("This looks totally illegal to me.")
+      flag_modal.check_confirmation
+
+      flag_modal.confirm_flag
+
+      expect(page).to have_content(I18n.t("js.post.actions.by_you.illegal"))
+    end
+  end
 end

spec/system/page_objects/modals/flag.rb

@@ -20,6 +20,14 @@ module PageObjects
         select_kit.expand
         select_kit.select_row_by_value(action)
       end
+
+      def fill_message(message)
+        body.fill_in("message", with: message)
+      end
+
+      def check_confirmation
+        body.check("confirmation")
+      end
     end
   end
 end