Order budget headings by id

To maintain order after editing a heading’s name

We should probably add timestamps to `groups` and `headings` and order
by `created_at` instead

Could have done it in a separate PR, but gotta keep moving to other
priority issues. Creating issue for timestamps to do correctly and with
tests 😌

app/views/admin/budgets/_groups.html.erb

@@ -14,6 +14,6 @@
 
 <% groups.each do |group| %>
   <div id="<%= dom_id(group) %>">
-    <%= render "admin/budgets/group", group: group, headings: group.headings %>
+    <%= render "admin/budgets/group", group: group, headings: group.headings.order(:id) %>
   </div>
 <% end %>