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/api-keys.html

@@ -29,7 +29,7 @@
     {{ current_service.id }}
   </p>
 
-  {% call(item) list_table(
+  {% call(item, row_number) list_table(
     keys,
     empty_message="You haven’t created any API keys yet",
     caption="API keys",