Removes use of slugs to edit group name

Changing a group’s `to_param` to return the slug instead of the id,
breaks many tests in the user facing interface

We should use slugs in upstream soon, but it should be done in a
separate PR, bringing the whole slug implementation from Madrid’s fork
and the corresponding test coverage

app/controllers/admin/budget_groups_controller.rb

@@ -3,13 +3,14 @@ class Admin::BudgetGroupsController < Admin::BaseController
   feature_flag :budgets
 
   def create
-    @budget = Budget.find params[:budget_id]
+    @budget = Budget.find(params[:budget_id])
     @budget.groups.create(budget_group_params)
     @groups = @budget.groups.includes(:headings)
   end
 
   def update
-    @group = @budget.groups.by_slug(params[:id]).first
+    @budget = Budget.find(params[:budget_id])
+    @group = @budget.groups.find(params[:id])
     @group.update(budget_group_params)
   end
 
@@ -19,8 +20,4 @@ class Admin::BudgetGroupsController < Admin::BaseController
       params.require(:budget_group).permit(:name)
     end
 
-    def load_budget
-      @budget = Budget.find_by(slug: params[:budget_id]) || Budget.find_by(id: params[:budget_id])
-    end
-
 end