Make regions collapsible on the block layout administration page

Problem/Motivation

Using many blocks on the block layout administration page (/admin/structure/block) on large sites makes administration very hard.

Being able to collapse regions would allow to hide long lists of blocks and gives a better overview on block regions. Especially if combined with an "un- / collapse all" functionality at the top.

Steps to reproduce

Proposed resolution

• Make regions collapsible to only show the region name, but hide the block listing
• Add a switch to open / collapse all regions in the top of the page

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet