doc: improve hub selinux instructions
    
    Make the Server HowTo SELinux instructions simpler and easier to read.
    Describe each step for users to copy-and-paste.
    
    Use semanage and restorecon instead of chcon in order to make the
    changes persist across relabling operations.
    
    Only recommend httpd_can_network_connect_db=1 if the user is using a
    TCP/IP connection to PostgreSQL. If koji-hub is using a local Unix
    socket connection, Apache does not need this setting.