Groups and headings CRUD from budget view

Before, users needed to navigate to the list of groups in order to
add, edit or delete a group.

Also, they need to navigate to the list of groups first, and then to
the list of headings for that group in order to add, edit or delete a
heading.

Now, it's possible to do all these actions for any group or heading
from the participatory budget view to bring simplicity and to reduce
the number of clicks from a user perspective.

Co-Authored-By: Javi Martín <javim@elretirao.net>

spec/controllers/admin/budget_groups_controller_spec.rb

@@ -1,12 +1,12 @@
 require "rails_helper"
 
 describe Admin::BudgetGroupsController, :admin do
-  describe "GET index" do
+  describe "GET new" do
     it "raises an exception when the feature is disabled" do
       Setting["process.budgets"] = false
 
       expect do
-        get :index, params: { budget_id: create(:budget).id }
+        get :new, params: { budget_id: create(:budget).id }
       end.to raise_exception(FeatureFlags::FeatureDisabled)
     end
   end