[UX] Make sidebars collapsible in Harris layout

Description of the need

Older versions of Drupal used a layout design style where if a sidebar does not contain any blocks, the sidebar would be collapsed by default.

Collapsing sidebars were implemented in Drupal for several reasons:

1) Way back when this pattern was adopted for Drupal, the average width of a browser was quite narrow (about 800px) and most sites wanted to take advantage of all the screen real-estate that was available.

2) A Drupal theme was limited to a single set of regions. A common way to accomplish building a site with multiple "Layout Templates" using only a single set of regions was to show or hide those regions on different pages. This could be achieved by controlling the visibility conditions on the blocks within those regions.

Many sites coming from Drupal 6 and 7 will require this older design style, and will not be satisfied with the more modern and user-friendly approach adopted by Backdrop back in 2014-2015.

Having a Layout option that behaves similarly to the Bartik theme may also be a familiar place for Drupal users to start, until they begin to understand the benefits of the Backdrop Layouts system.

Proposed solution

Layout in contrib https://github.com/backdrop-contrib/harris_flexible has minor changes to css and html that accomplish this task. I hope that this code can be merged into core code.

Alternatives that have been considered

Currently there is layout in contrib that accomplishes exactly this task https://github.com/backdrop-contrib/harris_flexible

Additional information

Draft of feature description for Press Release (1 paragraph at most)

Backdrop new Harris Layout does not show empty sidebars