API: allow to edit issue milestone
    
    New API endpoint has been added, see specification bellow
    
    """
    Change issue milestone
    -------------------
    Change the milestone of an issue.
    
    ::
    
         POST /api/0/<repo>/issue/<issue id>/milestone
         POST /api/0/<namespace>/<repo>/issue/<issue id>/milestone
    
    ::
    
         POST /api/0/fork/<username>/<repo>/issue/<issue id>/milestone
         POST /api/0/fork/<username>/<namespace>/<repo>/issue/<issue id>/milestone
    
    Input
    ^^^^^
    
    +----------------- +---------+--------------+------------------------+
    | Key              | Type    | Optionality  | Description            |
    +==================+=========+==============+========================+
    | ``milestone``    | string  | Optional     | The new milestone of   |
    |                  |         |              | the issue, can be any  |
    |                  |         |              | of defined milestones  |
    |                  |         |              | or empty to unset the  |
    |                  |         |              | milestone              |
    +----------------- +---------+--------------+------------------------+
    
    Sample response
    ^^^^^^^^^^^^^^^
    
    ::
    
        {
          "message": "Successfully edited issue #1"
        }
    
    """
    
    Fixes: https://pagure.io/pagure/issue/2111