Allow searching the content of the comments on an issue tracker
    
    Using ``content:<keyword>`` one can now search all the comments on the
    issue tracker for that keyword.
    This works but it slow and thus should not be abused, so it's in
    purpose not being proposed prominently to the users.
    
    Fixes https://pagure.io/pagure/issue/1749
    
    Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>