Update the RTD integration to rely on the new /v2/ API
    
    We're now asking the users for:
    - an API URL provided by RTD
    - an API token provided by RTD
    - a list of branches that should trigger a build
    
    Fixes https://pagure.io/pagure/issue/2522
    
    Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>