Relabel show more links on the dashboard

- ‘messages sent per month’ is a better description of what will appear
  on the page than ‘activity breakdown’
- ‘templates used by month’ instead of ‘this year’ for consistency

I think ‘usage breakdown’ still works for the remaining link – it’s more
than a monthly breakdown, it also breaks down the spend.

Doesn’t need to say ‘by month’ in the `<h1>`s themselves, because you
can see from looking at the page that it’s broken down by month.

app/templates/views/dashboard/monthly.html

@@ -6,14 +6,14 @@
 {% extends "withnav_template.html" %}
 
 {% block service_page_title %}
-  Activity breakdown
+  Messages sent,
   {{ selected_year }} to {{ selected_year + 1 }} financial year
 {% endblock %}
 
 {% block maincolumn_content %}
 
   <h1 class="heading-large">
-    Activity breakdown
+    Messages sent
   </h1>
   <div class="bottom-gutter">
     {{ pill(