Fixes#4071.
Major changes:
- the bridge's URI changed from zen.yandex.com to dzen.ru, as the former
redirects to the latter (perhaps the bridge's name should be changed
as well);
- the channel's URL is now required instead of the channel's username;
- two kinds of URLs are supported, one for channels with usernames and
one for channels with IDs in their URL;
- the channel's real name, as shown in the webpage, is now used as the
feed title.
