Change BudgetInvestmentStatus to Milestone::Status

Generalize the BudgetInvestmentStatus model to Milestone::Status so it
is not specific to budget investments, but can be used for any entity
which has milestones. This is in preparation to make the Milestone
model polymorphic and usable by entities other than budget investments.

config/routes/admin.rb

@@ -69,7 +69,7 @@ namespace :admin do
     resources :budget_phases, only: [:edit, :update]
   end
 
-  resources :budget_investment_statuses, only: [:index, :new, :create, :update, :edit, :destroy]
+  resources :milestone_statuses, only: [:index, :new, :create, :update, :edit, :destroy]
 
   resources :signature_sheets, only: [:index, :new, :create, :show]