Includes:
- turning off :visited styles to match existing
design
- swapping heading classes used to make links bold
for the GOVUK Frontend bold override class
- adding visually hidden text to some links to
make them work when isolated from their context
We may need to revisit whether some links, such as
those for documentation and features, may benefit
from having some indication that their target has
been visited.
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.
