[frontend] support ?priority=x for non copr:// repo
    
    This is an alternative to PR#539
    
    Initially, I thought, that it will be enough to remove the
    copr:// condition from `parse_repo_params` to get the
    special params dict and simultaneously keep ?priority=x
    in the repo baseurl. However, although baseurl with ?priority
    can be opened in the browser, DNF can't handle it, so we need
    to strip it.
    
    Theoretically, false positives can occur, but IMHO it is
    not a real threat.