Add groups index page

When render the investment list component with the link "see all
investments", now we redirect to groups index page when a budget has
multiple headings.

config/routes/budget.rb

@@ -1,5 +1,5 @@
 resources :budgets, only: [:show, :index] do
-  resources :groups, controller: "budgets/groups", only: [:show]
+  resources :groups, controller: "budgets/groups", only: [:show, :index]
   resources :investments, controller: "budgets/investments" do
     member do
       put :flag