UX: include a flag reason in the post-deleted-by-staff-because-of-flags message

app/models/post.rb

@@ -436,8 +436,12 @@ class Post < ActiveRecord::Base
     post_actions.where(post_action_type_id: PostActionType.flag_types_without_custom.values, deleted_at: nil).count != 0
   end
 
+  def active_flags
+    post_actions.active.where(post_action_type_id: PostActionType.flag_types_without_custom.values)
+  end
+
   def has_active_flag?
-    post_actions.active.where(post_action_type_id: PostActionType.flag_types_without_custom.values).count != 0
+    active_flags.count != 0
   end
 
   def unhide!

config/locales/server.en.yml

@@ -2253,6 +2253,8 @@ en:
 
         <%{base_url}%{url}>
 
+        %{flag_reason}
+
         This post was flagged by the community and a staff member opted to remove it.
 
         Please review our [community guidelines](%{base_url}/guidelines) for details.

lib/post_destroyer.rb

@@ -201,7 +201,10 @@ class PostDestroyer
         :send_system_message,
         user_id: @post.user.id,
         message_type: :flags_agreed_and_post_deleted,
-        message_options: { url: @post.url }
+        message_options: {
+          url: @post.url,
+          flag_reason: I18n.t("flag_reasons.#{@post.active_flags.last.post_action_type.name_key}", locale: SiteSetting.default_locale)
+        }
       )
     end