Add a username slug to the user URL

This way it won't be possible to browse all user URLs by just going to
/users/1, /users/2, /users/3, ... and collect usernames, which might not
be desirable in some cases.

Note we could use the username as a URL parameter and just find the user
with `@user = User.find_by!(id: id, username: username)`, but since
usernames might contain strange characters, this might lead to
strange/ugly URLs.

Finally, note we're using `username.to_s` in order to cover the case
where the username is `nil` (as is the case with erased users).

app/controllers/direct_messages_controller.rb

@@ -1,6 +1,7 @@
 class DirectMessagesController < ApplicationController
   before_action :authenticate_user!
   load_and_authorize_resource :user, instance_name: :receiver
+  before_action :check_slug
   load_resource through: :receiver, through_association: :direct_messages_received
   authorize_resource except: :new
 
@@ -28,6 +29,12 @@ class DirectMessagesController < ApplicationController
 
   private
 
+    def check_slug
+      slug = params[:user_id].split("-", 2)[1]
+
+      raise ActiveRecord::RecordNotFound unless @receiver.slug == slug.to_s
+    end
+
     def direct_message_params
       params.require(:direct_message).permit(allowed_params)
     end