Add comment reactions
    
    A user can select from a predefined list of reactions for a comment.
    There can be multiple reactions from the same person on a single
    comment, but only one of each type.
    
    The available reactions are listed in the config file as a human
    readable name (which will ultimately be stored in database) and a
    emojione class used for displaying.
    
    This works for both issues and pull requests.
    
    The reactions are visible in the API, but can only be posted via the web
    UI.
    
    Fixes: https://pagure.io/pagure/issue/812