adding milestone/current api endpoint. Fixes T328
    
    Summary: adding milestone/current api endpoint as a convenience method and a .simple() dict representation of milestones
    
    Test Plan: added unit test, tested locally in dev env
    
    Reviewers: mkrizek
    
    Subscribers: tflink
    
    Differential Revision: https://phab.qadevel.cloud.fedoraproject.org/D230