Updates the help center documentation on subscribing users to
streams to include description for doing so via mentioning a
user while composing a message.
Fixes#21796.
Adds a new help center article focused on managing a single
user's stream subscriptions.
Creates a shared file for instructions to navigate to a user's
full profile via the right sidebar, which is used in three help
center articles.
Fixes#21795.
Extends the documentation on unsubscribing users from streams to
include an alternate method via the user's full profile, which
is useful for cases where admins may need to unsubscribe a single
user from multiple streams.
Fixes#21379.