bodhi-backend: Add zchunk support to updates and updates-testing repositories
    
    This re-adds zchunk support for the updates and updates-testing repositories
    for both rpms and modularity.
    
    Zchunk metadata was turned off due to a broken version of librepo that made it
    out to stable, but a fixed version has been pushed and FESCo has decided[1] to
    go ahead and turn this back on.
    
     1: https://pagure.io/fesco/issue/2116
    
    Signed-off-by: Jonathan Dieter <jdieter@gmail.com>