Fix 500 error from notifications/letter.json page
Fix 500 error from notifications/letter.json page

The letters notification page makes ajax calls to update the status - these
were failing because it's making a POST request without a CSRF token.
(The email and SMS notification pages contain a search form that includes
the hidden CSRF input, so this was only occurring with the letters
page.)

This commit adds a hidden form for the letters page which just contains
the CSRF token.

gov-cjwaszczuk/notifications-admin,gov-cjwaszczuk/notifications-admin,alphagov/notifications-admin,alphagov/notifications-admin,alphagov/notifications-admin,alphagov/notifications-admin,gov-cjwaszczuk/notifications-admin,gov-cjwaszczuk/notifications-admin