MDL-37178 quiz attempts: make it easy to add buttons to the UI.

There are two buttons which I think in general it is better not to have
in the quiz UI:

1. A previous button, beside the Next button.

2. A Save changes (while remaining on the same page) button.

While in general I think it is better not to have these buttons, some
people have good reasons why in their situation, the buttons are a good
idea. Therefore, this change adds the necessary code to the renderer and
elsewhere, to make it really easy to add these buttons by overriding the
renderer in a theme.