feat(menu): simplify menu expand/pin control

See #1498 (closed). This is a quick experiment for consideration, based on branch 1498-minimise-sidebar. This removes the chevron control, reduces the space used by the expansion bar, and makes the whole bar clickable to pin. Once pinned, the bar disappears, leaving only the pinned sidebar icon.

I don't think the styling's quite perfect and some cleanup might be in order: for example it no longer makes sense to use a Button for the pin icon.