Add many-2-many relationship between ArtifactBuild and Compose
    
    This is used to refactor storing ODCS compose IDs for artifact builds
    that will be rebuilt.
    
    For a particular event to rebuild series of images, this new database
    schema change will be able to describe like this, an event will rebuild
    several images, and each build has ODCS composes containing
    updated RPMs.
    
    We can get all composes IDs through this many-2-many
    relationship instead of gathering them from Event.compose_id and
    ArtifactBuild.build_args. Meanwhile, it will be easy and flexible for
    adding more composes IDs to a build without changing schema.
    
    Signed-off-by: Chenxiong Qi <cqi@redhat.com>