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/manage-users.html

@@ -29,7 +29,7 @@ Manage users – GOV.UK Notify
     {% endif %}
   </div>
 
-  {% call(item) list_table(
+  {% call(item, row_number) list_table(
     users, caption='Active', **table_options
   ) %}
     {% call field() %}
@@ -48,7 +48,7 @@ Manage users – GOV.UK Notify
   {% endcall %}
 
   {% if invited_users %}
-    {% call(item) list_table(
+    {% call(item, row_number) list_table(
       invited_users, caption='Invited', **table_options
     ) %}
       {% call field() %}