frontend, python: deduplicate APIv3 build-chroot parameters
    
    We were sending `build_id` and `chrootname` values as a part of the
    URL path as well as query parameters. I am dropping them from the URL
    path, they are unnecessary there (and IMHO not really used). I am
    updating the route so it is accessible via the current URL as well as
    the new simple one. So, at this point, both old URL, e.g.
    
        /api_3/build-chroot/1341930/fedora-rawhide-x86_64?build_id=1341930&chrootname=fedora-rawhide-x86_64
    
    and new URL
    
        /api_3/build-chroot?build_id=1341930&chrootname=fedora-rawhide-x86_64
    
    will work. At the same time, I am updating the python-copr client to
    use the new simplified URL since it is the prefered one (other GET
    endpoints in our API, e.g. for getting projects or builds also doesn't
    specify any values in the URL path but use query parameters instead).
    
    Fixes: #1352
    Merges: #1573