Add govukCheckboxesField
Add govukCheckboxesField

govukCheckboxesField subclasses
SelectMultipleField and overwrites how it renders
HTML to let us use the GOVUK Checkboxes component
while retaining all the functionality of WTForms
fields.

Based on work on github.com/richardjpope/recourse:

https://github.com/richardjpope/recourse/blob/master/recourse/forms.py#L6

alphagov/notifications-admin,alphagov/notifications-admin,alphagov/notifications-admin,alphagov/notifications-admin