XWIKI-9738: Finalize migration from workspaces to the new wiki API.

* Test if XWiki.ManagerWorkspace exists instead of Panels.WorkspaceInformationPanel (might be better, there is less probability that the user has removed that technical page than a panel).
Merge pull request #237 from gdelhumeau/XWIKI-9738

XWIKI-9738: Finalize migration from workspaces to the new wiki API.