* Move content on moving topics between streams to a dedicated
article. We advertise it as "move content" to hint that one can move
messages or split topics, and link to it.
* This deletes change-the-topic-of-a-message, because the same content
is already covered in rename-a-topic.
* This commit mostly just moves content between articles. Most of that
content was redundant with the first few paragraphs of the surviving
"rename a topic" article. The former "This is useful for" se ntence
was adapted to the remaining article.
* This commit also adds a redirect for the removed article, and
updates related links.
Damian Parrino