Add bounding boxes for clusters

[wbazant]

Follow-up from falling-fruit/falling-fruit-web#423. Going to isolated clusters could be a nicer experience if we were able to zoom in just into their bounding box.

Implementation idea, without looking at the code: given a list of locations for a cluster and their centre, find max(lat), min(lat), max(lng), min(lng)?

The bounding boxes could be calculated optionally, maybe even with a hardcoded upper bound for a count, since they're most useful for sparsely populated places with a handful of locations.

For example: two locations in Ullapool town centre are visible as a cluster in https://beta.fallingfruit.org/map/@58.1014684,-5.5723643,8z but it takes three clicks to get to them.