FIX Not checking stage in SiteTree#canView

SiteTree versions that arent the live version shouldnt be accessed by
regular users, but the logic to check that was split off into canViewStage,
which wasnt checked by code that isnt specifically SiteTree aware
(like RestfulServer)
Merge pull request #792 from hafriedlander/fix/stageaccess

FIX Not checking stage in SiteTree#canView