Allow editing the budget in budget creation wizard

When users created a budget and made a typo, they could use the link to
go back to edit a budget. However, after doing so, they were out of the
budget creation process.

So we're now letting users go back to edit the budget, fix any mistakes
they might have made, and then continue to groups.

config/routes/admin.rb

@@ -73,7 +73,7 @@ namespace :admin do
   end
 
   namespace :budgets_wizard do
-    resources :budgets, only: [:create, :new] do
+    resources :budgets, only: [:create, :new, :edit, :update] do
       resources :groups, only: [:index, :create, :edit, :update, :destroy] do
         resources :headings, only: [:index, :create, :edit, :update, :destroy]
       end