Specify group_by_aggregation (fixes EIS-507)

group_by_aggregation no longer defaults to django.db.models.Sum, which isn't
the correct aggregation for WholeBuildingPower, anyway.
Remove limit on heat map

Heat map height is now dynamic so all rows are visible, and grouping by hour is
fixed so we no longer have browser rendering performance issues caused by the
presence of multiple values for each hour. Heat map now displays a year's worth
of data with minimal (~1 second) rendering time.