[misc] Bulletproofing.
XWIKI-10186: Duplicated root namespace extension in DW "Extensions" step
* Fix left-over usage of upgrade mode.
XWIKI-10210: Extension Updater should let global admins choose whether to compute the upgrade plan for the entire farm or just for the main wiki
XWIKI-10211: Paginate the Wikis and Extensions steps from Distribution Wizard