Instead of referring to users being added or removed from channels,
we now use subscribe or unsubscribed from channels.
Splits the article for adding and removing users from a channel
into separate articles: /help/subscribe-users-to-a-channel and
/help/unsubscribe-users-from-a-channel.
The URL redirects for the combined add/remove articles (for both
channel and stream terminology) go to the subscribe users to a
channel article.
Updates the content in `help/include/create-a-channel-instructions`
and `help/include/create-channels-intro` for the stream to channel
rename. Done in a separate commit from the previous so that the
rename of these files is tracked as such in the history.
