[Proposal] Add an assertion that checks whether a view lacks a given variable. (Was against master branch)

I am writing a RESTful API, in which I have groups that entities are attached to. These groups are themselves attached to a parent company, which the admin user is also attached to. So no need to specify the parent company. But I also have a super admin, who can move a group to another parent company.

This assertion allows me to check that the parent companies list is not sent to the view when the user is not super admin level. I have written it for myself, but I imagine there are other situations when it would be useful, so I propose to include it into the framework.