Upgrade task to ensure that for all the user names in Confluence a ConfluenceUser object
exist. As well as taking the user names from the Crowd tables, the various username related columns
across all content table are considered. This is consistent with Confluence's behaviour of allowing
content to be owned/created/modified/whatever by non-existent users.
Note that various "post schema" upgrade tasks assume this task has already run and make the assumption
that all content referenced users as well as crowd users (which may not yet have associated content)
are populated in the user_mapping table (ConfluenceUser). e.g.
UserIndexingUpgradeTask
.