Change ModuleBuild.context to db column
    
    1. Changed ModuleBuild's context property to db column, it's
    non-nullable and default value is '00000000' to keep it consistent
    with previous behaviour.
    
    2. Changed ModuleBuild.contexts_from_mmd to return a tuple of
    (ref_build_context, build_context, runtime_context, context).
    
    3. Updated tests affected by this change.