Use same structure on new and edit budget forms

The margin between the heading and the form was different due to margin
collapsing, which applied in one case but not the other.

app/assets/stylesheets/admin/budgets/form.scss

@@ -5,6 +5,10 @@
     clear: both;
     margin-top: $line-height * 1.5;
 
+    &:first-of-type {
+      margin-top: 0;
+    }
+
     legend {
       color: $admin-text;
       font-size: $small-font-size;

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

@@ -1,3 +1,7 @@
+<%= back_link_to admin_budgets_path %>
+
+<header>
   <h2><%= t("admin.budgets.new.title") %></h2>
+</header>
 
 <%= render "/admin/budgets/form" %>