Switch locality sorting to `wof:population_rank`

[trescube]

wof:population_rank for WOF is now available for the vast majority for localities as of https://github.com/whosonfirst-data/whosonfirst-data/pull/891/files. This module currently uses bands based on somewhat arbitrary raw population numbers from wof:population whereas we'd probably be better served to use the 1-15 range from wof:population_rank for sorting localities.

[trescube]

Waiting on whosonfirst-data/whosonfirst-data#824 to be merged. See https://github.com/whosonfirst/whosonfirst-properties/tree/master/properties/wof#population_rank for value definitions.