Allow searching the content of the comments on an issue tracker
    
    Using ``content:<keyword>`` one can now search all the comments on the</keyword>
    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></pingou@pingoured.fr>