Enforce minimum version for Safe Exam Browser

This is a quiz access rule plugin that allows teachers to enforce a minimum version of the Safe Exam Browser for their quiz. The required version can be set side-wide by the administrators. If a student uses an outdated version of the Safe Exam Browser, the quiz will be covered by a modal overlay, preventing them from answering questions.

Note that the verification is done in the student's browser, so it is prone to manipulation. However, in a properly configured, official build of the Safe Exam Browser, users do not have access to developer tools like the Javascript console or the DOM inspector, so they should not be able to sidestep the checks.