Per @Legoktm:
- AbortMove has been removed completely.
- MovePageIsValidMove should only be used for technical restrictions. NS_TOPIC qualifies here.
- However, for permissions issues (is the destination namespace Flow and if not do they have flow-create-board), we should use MovePageCheckPermissions.
While we're at this, document these hooks on-wiki (it's currently only in docs/hooks.txt)