Updates "help/manage-a-user" links that are contextually for general
information about users to go to the new "help/introduction-to-users"
article.
Adds a link to "Manage a user" in the related articles section of
the new intro to users 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.
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.
