Improve the browse page's UI

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

acteng / atip