Use a <caption> for the template statistics table

This is semantically more accurate because it’s describing the whole
table, not just the first column.

Adjusting the font-size to make it sit within the ‘In the last 7 days’
section. Adjusting the spacing because now that we have more borders we
don’t need quite so much whitespace to separate different bits of the
page.

app/templates/views/dashboard/template-statistics.html

@@ -9,14 +9,14 @@
     <div class='template-statistics-table'>
       {% call(item, row_number) list_table(
         template_statistics,
-        caption="Templates used",
-        caption_visible=False,
+        caption="By template",
+        caption_visible=True,
         empty_message='',
         field_headings=[
           'Template',
           'Messages sent'
         ],
-        field_headings_visible=True
+        field_headings_visible=False
       ) %}
 
         {% call row_heading() %}