It should not be possible to assign approvers on page for people who do not have view right

There's currently no check performed on the approvers added to a page, so it's possible to assign to a page an approver who do not have view right on that page.
The impact is that if a strategy is used that requires all approvers to review CR, those CR couldn't ever be merged since the approvers couldn't access it.

So we need a mechanism to ensure that the approvers set on a page have view right, and we need to be sure that it's also checked when requesting for page creation with setting approvers.