WIKI-706 [Migration][Unified Search] Sandbox Wiki Pages in Search results

Problem analysis:
* In Platform 3.5, sandbox wiki pages are generated automatically as group wiki pages.
  The initialization of such wiki no longer exist on Platform 4.0. Its pages are therefore inaccessible.

Fix description:
* Implement an upgrade plugin named RemoveSandboxPageUpgradePlugin to remove all sandbox pages.
Merge commit '6bd3b8151a09db8c9e1df8755ca1adc30ed1245a' into HEAD