Add row numbers to tables

When a table is showing the contents of a CSV file, it should look
something like a spreadsheet.

The minimally skeuomorphic way to do this is by adding row numbers.

This commit doesn’t
- make the row numbers monospace (it’s barely noticeable and doesn’t
  reflect what actual spreadsheets do)
- make the first column heading ‘Row’ (again, doesn’t reflect how actual
  spreadsheets work, and takes up more valuable space)

app/templates/views/notifications.html

@@ -20,7 +20,7 @@
       <a href="{{ url_for(".view_notifications", service_id=current_service.id, status=['sent', 'delivered'], page=1) }}">Successful messages</a>&emsp;
       <a href="{{ url_for(".view_notifications", service_id=current_service.id, status=['failed', 'complaint', 'bounce'], page=1) }}">Failed messages</a>
     </p>
-    {% call(item) list_table(
+    {% call(item, row_number) list_table(
       notifications,
       caption="Recent activity",
       caption_visible=False,