DEV: mount `DiscourseDev` engine routes at `/dev` path. (#12376)

Gemfile.lock

@@ -114,7 +114,7 @@ GEM
       railties (>= 3.1)
     discourse-ember-source (3.12.2.3)
     discourse-fonts (0.0.8)
-    discourse_dev (0.0.4)
+    discourse_dev (0.0.5)
       faker (~> 2.16)
     discourse_image_optim (0.26.2)
       exifr (~> 1.2, >= 1.2.2)

config/routes.rb

@@ -963,6 +963,10 @@ Discourse::Application.routes.draw do
     post "/do-not-disturb" => "do_not_disturb#create"
     delete "/do-not-disturb" => "do_not_disturb#destroy"
 
+    if Rails.env.development?
+      mount DiscourseDev::Engine => "/dev/"
+    end
+
     get "*url", to: 'permalinks#show', constraints: PermalinkConstraint.new
   end
 end