Added a send button which only appears on the page if the query string
indicates that the PDF is valid. Before actually sending, we check that
the service has the right permissions and that the metadata for the
letter confirms the letter is valid (because the query string can be
changed).
This sanitises uploaded letters and stores the sanitised result in S3
with if it passes validation or the original PDF in S3 if validation
fails. A metadata value of 'status' is set to either 'valid' or
'invalid'.
Added a form to upload a single letter. Currently this only uses the
form to validate that a file is submitted and that the file is a PDF. If
either of these validations fail, the form will display an error.
Otherwise, we redirect to a new preview page which just has the filename
as the heading for now.
