Histogram tiles are untenable at scale

As a consequence of #150, it is now known that the edit histogram tile approach doesn't scale well. After running on OSM, many large tiles were outputted. These are multimegabyte tiles, where the guidelines (referred to [here](https://stackoverflow.com/questions/40819377/bypass-mapbox-500kb-individual-tile-size-limit), link to mapbox docs is broken) suggest (and possibly enforce) a 500kb file size limit. This may be leading to apparently broken demos where tiles exist, but are not being displayed.

It might be time for a ground-floor rethink of the representation of these histograms to allow for more parsimonious tiles.

Start with issue #150 and trace the edit histogram tile approach that produced the OSM output. Measure the generated tile sizes and inspect why demos fail to display large tiles. Done means the histogram representation is substantially more parsimonious and avoids the reported tile-size limit.

Written by the indexing model from the issue text.