Make task list read better for screen readers

This duplicates how the task list pattern is coded in the GOV.UK
Prototype kit[1]. It adds ARIA attributes and the use of a
semantically-meaningful element (`<strong>`) to give more information to
screen reader users.

1. https://govuk-prototype-kit.herokuapp.com/docs/templates/task-list

app/utils.py

@@ -206,6 +206,10 @@ def email_safe(string, whitespace='.'):
     return string.strip('.')
 
 
+def id_safe(string):
+    return email_safe(string, whitespace='-')
+
+
 class Spreadsheet():
 
     allowed_file_extensions = ['csv', 'xlsx', 'xls', 'ods', 'xlsm', 'tsv']