Fix links for file attachment URLs on CDNs in e-mails.
File attachment URLs on CDNs weren't showing up correctly in e-mails. We

were prepending the http(s)://domain to them, which was a problem when

that was already included in the URL. We now guarantee that

FileAttachment.get_absolute_url will return an actual absolute URL,

domain included, and it will only prepend the http(s)://domain if it's

not already there.

Checked an e-mail for a file not on a CDN, and a file simulated to be on

a CDN. Both links looked correct.