403 error improvements

[watucker]

Need a better 403 page for when you arrive at a restricted download page on dap.ceda.ac.uk. This could be a redirect back to the archive browser (would be nice to leverage the templates from this codebase). Some work is needed on the Nginx configuration for dap.ceda.ac.uk to make sure the redirect happens.

If the redirect goes back to the same page they were when the user clicked to download a file, this would be a great user experience.

[watucker]

Can use URL parameter on data.ceda.ac.uk to indicate a login failure at a path. Possibly, "?forbidden"

[spepler]

@Adik8688 this is related to the authorisation problem so assigning you here.