Accessibility: Improve reordering of the post boxes in the block editor meta boxes area.

Follow-up to [48373].
- ignores hidden "sortables" areas 
- hides the reorder buttons when there's only one post box 
- makes the reorder buttons slightly bigger to match the side of the toggle button

Fixes #39074.

Built from https://develop.svn.wordpress.org/trunk@48460


git-svn-id: https://core.svn.wordpress.org/trunk@48229 1a063a9b-81f0-0310-95a4-ce76da25c4cd