URL-decode the address in dclogin URLs #7943

iOS forcefully encodes dclogin URLs clicked in the browser, and this results in URLs that look like:
dclogin://foobar%40example.org... instead of dclogin://foobar@example.org.

This was a problem for chatmail/relay#855 already, there the problem is fixed with return f"dclogin:{quote(email, safe='@')}?p={quote(password, safe='')}&v=1&ic=3" which prevents @ from being URL-escaped. But if iOS user clicks such the link, @ gets escaped anyway and this breaks.

Also dclogin://foobar%40%5b192.168.1.1%5d should be tested, address should be decoded the same as dclogin://foobar@[192.168.1.1], there is no difference.