A seafile server can be configured to use a relative URL as
FILE_SERVER_ROOT in order to support more than one hostname/ip. (see
https://github.com/haiwen/seahub/issues/3398#issuecomment-506920360 )
The previous backend implementation always expected an absolute
download/upload URL, resulting in an "unsupported protocol scheme"
error.
With this commit it supports both absolute and relative.