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'.
This checks that the PDF file is not malformed in some way (e.g. by
missing the EOF marker). We check this by trying to get the page count
of the letter which will be needed to display the preview of the letter.
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.
The uploads hub is just a page with text for now - there are no actions
available on the page. It is linked to from a new 'Uploads' menu item on
the left of the page which is only visible if your service has the
`letter` and `upload_letters` permissions and if the current user has
permissions to send messages.
