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/users_controller.rb

@@ -1,5 +1,6 @@
 class UsersController < ApplicationController
   load_and_authorize_resource
+  before_action :check_slug
   helper_method :valid_interests_access?
 
   def show
@@ -8,6 +9,12 @@ class UsersController < ApplicationController
 
   private
 
+    def check_slug
+      slug = params[:id].split("-", 2)[1]
+
+      raise ActiveRecord::RecordNotFound unless @user.slug == slug.to_s
+    end
+
     def valid_interests_access?(user)
       user.public_interests || user == current_user
     end