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/partials/jobs/notifications.html

@@ -1,6 +1,6 @@
 {% from "components/table.html" import list_table, field, right_aligned_field_heading %}
 
-{% call(item) list_table(
+{% call(item, row_number) list_table(
   notifications,
   caption=uploaded_file_name,
   caption_visible=False,