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.

app/models/post.rb

@@ -550,10 +550,14 @@ class Post < ActiveRecord::Base
   end
 
   def is_flagged?
+    flags.count != 0
+  end
+
+  def flags
     post_actions.where(
       post_action_type_id: PostActionType.flag_types_without_custom.values,
       deleted_at: nil,
-    ).count != 0
+    )
   end
 
   def reviewable_flag