Break out cluster groups

[neilrqm]

Currently, areas are listed linearly and it's a pain to change from one group of areas to another. But the API includes the cluster group names in its stats response! Instead of having big, flat list of areas, we want to break them out according to cluster groups. Each cluster group should be represented at the root of the area list and be able to expand to list the clusters/neighbourhoods in that group. Each cluster group should have a checkbox that can be used to select/unselect all areas in that group. The checkbox should also make use of its third state to indicate when some but not all of the group's areas are selected.