Use JavaScript pluralize for budget staff

We were using two different systems to set translations in JavaScript:
to set the text for languages, we were using data attributes, and to set
the text for staff members, we were using AJAX calls.

I find data attributes keep the code more simple, since there's no need
to define an extra route and controller action. Furthermore, the user
experience is better because response times are faster.

So now both places use data attributes.

config/routes/admin.rb

@@ -54,7 +54,6 @@ namespace :admin do
   resources :budgets do
     member do
       put :calculate_winners
-      get :assigned_users_translation
     end
 
     resources :groups, except: [:show], controller: "budget_groups" do