Provide /media urls for Documents

Cause Document object urls to point to /media/filename instead of to /document/doc_id/filename.

This is a workaround for a wart in wagtail that insists on serving documents as attachments.
It is a mild compromise in that the purported inside wagtail features of counting document references and controlling access to files is circumvented. However, by design, we don't want different access controls to files for different viewers of the IETF website, and we are looking at different ways to gather information like references.

See wagtail/wagtail#4359, wagtail/wagtail#1158, and the knot at wagtail/wagtail#1420.