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.

app/controllers/admin/budgets_wizard/budgets_controller.rb

@@ -8,16 +8,27 @@ class Admin::BudgetsWizard::BudgetsController < Admin::BaseController
   def new
   end
 
+  def edit
+  end
+
   def create
     @budget.published = false
 
     if @budget.save
-      redirect_to admin_budgets_wizard_budget_groups_path(@budget), notice: t("admin.budgets.create.notice")
+      redirect_to groups_index, notice: t("admin.budgets.create.notice")
     else
       render :new
     end
   end
 
+  def update
+    if @budget.update(budget_params)
+      redirect_to groups_index, notice: t("admin.budgets.update.notice")
+    else
+      render :edit
+    end
+  end
+
   private
 
     def budget_params
@@ -29,4 +40,8 @@ class Admin::BudgetsWizard::BudgetsController < Admin::BaseController
 
       valid_attributes + [translation_params(Budget)]
     end
+
+    def groups_index
+      admin_budgets_wizard_budget_groups_path(@budget)
+    end
 end