Allow quizaccess plugins to display extra HTML on quiz view page when quiz access is prevented.

Presently, when a quiz is prevented due to a quiz access rule, the sub-plugin responsible is able to supply a message string that will be displayed explaining why the quiz is restricted.

It would also be desirable to allow the sub-plugin to supply a block of HTML that may contain extra actions for the user to take in order to access the quiz.

The use case that prompted this, is to provide a download button for a Safe Exam Browser config file, and a link to download the Safe Exam Browser application.