Add the date_modified to projects API
    
    This commit add the date_modified timestamp to the projects API,
    the date_modified timestamp is updated whenever the project's settings,
    project's group settings or the project's user settings are modified.
    
    Fixes #2412
    
    Signed-off-by: Clement Verna <cverna@tutanota.com></cverna@tutanota.com>
    Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr></pingou@pingoured.fr>