[UX] Permissions page: Make permission groups rows collapsible/expandable.

This is a request to actually bring Better Permissions module functionality in core:

The module is fairly simple, but was never ported to D7/D8 (https://www.drupal.org/node/937208), so the UI in the screenshot might not impress at first.

I think the addition of a global expand/collapse link somewhere at the top + making each permission group expandable/collapsible would be great. I'm not sure about the way Better Permissions expands/collapses rows though... - when all rows are collapsed, selecting a row expands it - but when selecting another row, the previously expanded one collapses - so only one expanded row at a time