Load subject types from configuration
    
    Adds classes for decision subject item and type.
    
    This makes the code more modular and allows to easily enable or disable
    subject type specializations. E.g. Fedora instances probably don't need
    to include `/etc/greenwave/subject_types/redhat-module.yaml`. Also
    `brew-build` can be removed as alias in `koji_build.yaml`.
    
    Fixes "koji-build" in tests to "koji_build".
    
    TODO:
    
    - Add documentation for the new configuration.
    
    Signed-off-by: Lukas Holecek <hluk@email.cz>