Workshop: use full page width when displaying contents

When I was implementing the Workshop 2.0, it seemed to be a good idea to have some UI widgets with width less than 100% (such as the planner tool or the grades report). It used to be more common in Moodle UI by that time.

The world has changed. It is better to use the screens more effectively these days. Having the workshop use the full width is a good start for further improvements of the UI (responsiveness, accessibility etc).