Invoice UI improvements. Renamed pages, added description, and added total number of events

RIGS/templates/RIGS/event_invoice.html

@@ -5,7 +5,8 @@
 
 {% block content %}
     <div class="col-sm-12">
-        <h2>Events for Invoice (£ {{ total|floatformat:2 }})</h2>
+        <h2>Events for Invoice ({{object_list|length}} Events, £ {{ total|floatformat:2 }})</h2>
+        <p>These events have happened, but paperwork has not yet been sent to treasury</p>
         {% if is_paginated %}
             <div class="col-md-6 col-md-offset-6 col-sm-12 text-right">
                 {% paginator %}