Improve title in admin headings index

With the word "headings" in it, it's a bit easier to know where we are.

We're also using a translation since not every language in the world
uses a "/" as a standard separator between two terms.

app/views/admin/budget_headings/index.html.erb

@@ -1,7 +1,7 @@
 <%= back_link_to admin_budget_groups_path(@budget), t("admin.budget_headings.index.back") %>
 
 <header>
-  <h2><%= "#{@budget.name} / #{@group.name}" %></h2>
+  <h2><%= t("admin.budget_headings.index.title", budget: @budget.name, group: @group.name) %></h2>
   <%= link_to t("admin.budget_headings.form.create"), new_admin_budget_group_heading_path %>
 </header>