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 "Related articles" sections after the revisions to the
channel permissions documentation, focusing on the articles in the
"Channel management" section of the help center.
