Prepare theme for Drupal Canvas

Problem/Motivation

Canvas needs a bit different philosophy on page.html.twig file structure. There should be less regions (usually header, content and footer is enough) and the content section region should not be wrapped into centering container, so Canvas can handle full-width edge-to-edge layouts natively, by using the section SDC in canvas editor properly. Section SDC is already shipped by theme.

Proposed resolution

When user turns the "Use Drupal Canvas for page templates in this theme." option on, it should automatically enable only regions that makes sense for canvas.

Content should stay wrapped into section SDC only if page is not rendered by canvas (utilize rendered_by_canvas variable).