Peer reviewers with one-click access cannot see due date

[joemull]

An editor reported a reviewer could not find the due date for the review anywhere. This can happen if one-click-access is turned on for reviewers, as the review request view does not include the date. If the review request email also does not include the due date, as is default, the reviewer won't see it anywhere.

We can:

• [x] Add the due date to the review request view, including the one-click endpoint
• [x] Add the due date to the default review request email template: {{ review_request.date_due }}
• [x] Add the template variable for due date to the documentation or help text for the review reqeuest template setting.
• [x] Where a review_assignment template has not been editted, overwrite it with the new value