[IMP] prevent edition of page if groups which current user doesn't have

If the current view uses @groups attributes (possibly in called templates),
the corresponding elements are rendered to a void (empty string in qweb). If
said user can edit the page, does so and saves a view section in which there's
a @groups to which he has no access, the element[@groups] is completely
removed from the template once saved, losing it.

If QWeb encounters an @groups to which the current user has no right during
rendering, have it request a no-RTE page, so the user can not RTE-edit the
page (or drop snippets in it).