Add API endpoints for listing pull requests for a user
    
    There is the requests page for a user that is currently
    available in the web UI and it lists all the pull requests
    for a user (across all projects in a pagure instance) that
    are either filed by the user, or actionable by the user. e.g:
    
    https://pagure.io/user/pingou/requests
    
    This commit adds API endpoints for the information in both of the
    tables that are included in the requests HTML view:
    
    api/0/user/<user>/requests/actionable
    api/0/user/<user>/requests/filed
    
    Signed-off-by: Ryan Lerch <rlerch@redhat.com>