Improve the browse page's UI

[dabreegster]

A bunch of things:

• Line up the help buttons on the right, and remove the odd background
• Group layer controls, add scrolling to the right sidebar, and make sure it doesn't cover up the bottom-right map controls
• Move the color controls for interventions to the left sidebar. The left is for scheme data, the right is for extra layers -- hopefully that's more clear.
• Change cursors to indicate clickable layers
• Fix a bug, so that clicking on multiple layers only does the topmost one
• Reorder layers on the browse page, so scheme data is on top
• Start the browse page focused on England, even before loading data

screencast.mp4

[dabreegster]

Moved this to lib/common and renamed to avoid clashing with the built-in JS Map