Redirect attachment download to view if accessed from external link

[garethrees]

Some articles, reports, etc link directly to the attachment "download" URL rather than the "view" URL.

Since #8916, this means that when you click on one of these links, instead of being taken to example.com/request/foo/response/1234/attach/1/information.pdf, the file just downloads without the visitor ending up on an Alaveteli site URL.

This is annoying as:

1. They probably weren't expecting clicking a link to initiate a file download
2. It's difficult to get back to the request page. Before, you'd just delete /response/1234/attach/1/information.pdf from the URL; now you've got to copy the URL to the clipboard, paste it somewhere and then do the deletion.
3. It reduces the promotion of the site – people clicking the link are less likely to see the domain name and get curious about how ATI was used to get the information.

What would be ideal is if a "download" URL is visited from an external site, we redirect to the "view" URL for that attachment (if available).