UX: show an enveloppe icon when a badge is used in messages

- the badge count now includes messages
- only show the message badges to admins

db/migrate/20180514133440_add_pm_topic_count_to_tags.rb

@@ -0,0 +1,5 @@
+class AddPmTopicCountToTags < ActiveRecord::Migration[5.1]
+  def change
+    add_column :tags, :pm_topic_count, :integer, null: false, default: 0
+  end
+end