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.
Renames file and redirects existing links to "help/user-roles" as
the previous content tracks to that current file's content.
Updates existing links (excluding ReadtheDocs) to go to either
"help/user-roles" or "help/manage-permissions" depending on which
help center article applies to the content.
In the help center and production documentation, replace links to
the "Getting your organization started with Zulip" guide to the
new "Moving to Zulip" guide.
This commit updates the term 'stream' to 'channel' in the section
header 'Configure who can move messages to another stream' in
'/help/restrict-moving-messages'.
Also, references to this section has been updated.
These files are not Jinja2 templates, so there's no reason that they needed
to be inside `templates/zerver`. Moving them to the top level reflects their
importance and also makes it feel nicer to work on editing the help center content,
without it being unnecessary buried deep in the codebase.
