WebUI: Handle assertion if multiple notifications are present
    
    If multiple notifications of the same type are shown at the same
    time, assertion works for only the first one. This change enables to
    search for notification's content in all shown notifications.
    
    Fixes: https://pagure.io/freeipa/issue/8641
    
    Signed-off-by: Michal Polovka <mpolovka@redhat.com>
    Reviewed-By: Michal Polovka <mpolovka@redhat.com>
    Reviewed-By: Serhii Tsymbaliuk <stsymbal@redhat.com>