Add `form.validate_on_submit` for sending uploaded letter
Add `form.validate_on_submit` for sending uploaded letter

Decided it was better to call this then not. This does rely on
the file_id not being corrupted so the file_id passed
into `uploaded_letter_preview` is valid but am taking that risk
given it should only change if a user is changing the form html.

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