Make Milestones general, and not specific to Budget Investments

Generalize the Budget::Investment::Milestone model to a
polymorphic Milestone model so it can be used for entities
other than Budget::Investment.

config/routes/admin.rb

@@ -62,7 +62,7 @@ namespace :admin do
     end
 
     resources :budget_investments, only: [:index, :show, :edit, :update] do
-      resources :budget_investment_milestones
+      resources :milestones, controller: 'budget_investment_milestones'
       member { patch :toggle_selection }
     end