Add singular descriptions for libraries

This lets us write nice interface copy like ‘Choose a local authority
from the local authorities library’.

app/templates/views/broadcast/areas-with-sub-areas.html

@@ -4,13 +4,13 @@
 {% extends "withnav_template.html" %}
 
 {% block service_page_title %}
-  {{ library.name }}
+  {{ page_title }}
 {% endblock %}
 
 {% block maincolumn_content %}
 
   {{ page_header(
-    library.name,
+    page_title,
     back_link=url_for('.choose_broadcast_library', service_id=current_service.id, broadcast_message_id=broadcast_message.id),
   )}}