Split/auto publish

LMS-11017 https://openedx.atlassian.net/browse/LMS-11017
Also, a bit further up in this file (L85, can't comment on it directly since it's not technically part of the diff), the reference to https://edx-wiki.atlassian.net/wiki/display/ENG/Mongostore+Data+Structure , I have a few curious questions:

1) It seems like we didn't migrate the wiki articles to new JIRA/Confluence automatically.  Are there plans to do that eventually?  And, should we at least move *this* specific article over to the new JIRA/Confluence, so that our whole community can view it?

2) In general, where *is* the preferred place for documentation of this sort?  Attaching it to an external wiki feels a little dicey to me, though Github's wiki is still pretty underwhelming—do we have any official/standard way of handling this?  ( @nedbat , @singingwolfboy )