Add a new API endpoint allowing to update watch status on a project
    
    With this people with the appropriate API token will be able to adjust
    their own watch status and instance-wide admins will be able to do it
    for any user.
    
    Fixes https://pagure.io/pagure/issue/3205
    Fixes https://pagure.io/pagure/issue/3174
    
    Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr></pingou@pingoured.fr>