When we proxy a media request to a remote server, add a query-param, which will
tell the remote server to 404 if it doesn't recognise the server_name.
This should fix a routing loop where the server keeps forwarding back to
itself.
Also improves the error handling on remote media fetches, so that we don't
always return a rather obscure 502.
This is so that a single MediaRepository can be shared across all
resources, rather than having a "copy" per resource.
In particular this allows us to guard against both the thumbnail and
download resource triggering a download of remote content at the same
time.
