DEV: move discourse_dev gem to the core. (#13360)

And get avatar images from `discourse_dev_assets` gem.

lib/faker/discourse.rb

@@ -0,0 +1,26 @@
+# frozen_string_literal: true
+
+require 'faker'
+
+module Faker
+  class Discourse < Base
+    class << self
+
+      def tag
+        fetch('discourse.tags')
+      end
+
+      def category
+        fetch('discourse.categories')
+      end
+
+      def group
+        fetch('discourse.groups')
+      end
+
+      def topic
+        fetch('discourse.topics')
+      end
+    end
+  end
+end