CMSPageHistoryViewerController doesn't try to block you from viewing a page you do not have read access on #216
Description
Steps to reproduce
- You are logged in with a non-admin user
- There's a page you do not have read access on
- You attempt to acces the page history by manually typing its URL (e.g.: /admin/pages/history/show/3)
Expected results: You get refuse access with a 403 error
Actual: CMSPageHistoryViewerController attempts to fulfil your request and throws a 500 error when it tries to call a method on a null form object.
