[PP-1] Make the contextual side panel resizable

I added more spacing around the resizing handle while fixing its scrolling in 🐛 No upward auto-scroll when components are dragged Needs review . That was the super easy part of this issue. 🙂

The additional spacing is now in a different MR under 🐛 Unable to scroll component props form Needs review , along with a rewrite of the layout for the contextual panel. We should probably land that first before we work on this issue.