FEATURE: Add new don't feed the trolls feature (#21001)

Responding to negative behaviour tends to solicit more of the same. Common wisdom states: "don't feed the trolls".

This change codifies that advice by introducing a new nudge when hitting the reply button on a flagged post. It will be shown if either the current user, or two other users (configurable via a site setting) have flagged the post.

spec/system/page_objects/components/composer.rb

@@ -43,6 +43,10 @@ module PageObjects
         composer_input.value == content
       end
 
+      def has_popup_content?(content)
+        composer_popup.has_content?(content)
+      end
+
       def select_action(action)
         find(action(action)).click
         self
@@ -83,6 +87,10 @@ module PageObjects
       def composer_input
         find("#{COMPOSER_ID} .d-editor .d-editor-input")
       end
+
+      def composer_popup
+        find("#{COMPOSER_ID} .composer-popup")
+      end
     end
   end
 end