Only show the subscribers list on demand
    
    This will speed up the page loading as the images will only be loaded
    on demand and not on page load, which for issues/PRs with lots of people
    subscribed will have quite a performance impact.
    
    Fixes https://pagure.io/pagure/issue/5012
    
    Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>