This way we have a URL we can give people that always points to the
latest version of the spec.
And it makes our code more Flask-idiomatic to be using `url_for` to be
generating a URL, rather than passing around a constant.
We now use the pattern of showing a box at the top of the page with the
error. The error message has a heading and can have additional details.
Error messages and the invalid pages get stored in the S3 metadata.
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.
