Make clusters clickable hurricane-response/response-theme#22

[leckman]

Description

Addresses issue #22

Makes clusters clickable - both on the shelters page and the distribution points page.
Code largely adapted from mapbox cluster documentation. Eventually, it would be nice to de-dupe code between those two layout pages.

On click, recenters map on clicked cluster centroid and zooms until the cluster splits up (either into more clusters or individual points).

Renamed ZOOM to INITIAL_ZOOM so it's obvious that variable is only used for initialization and doesn't store the state of the current zoom. Cleaned up some whitespace in the file.

List of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• This pull request relates to a ticket.
• This code is tested.
• I want my code added to the response theme.

[leckman]

@nihonjinrxs not sure if you're the right person to tag - any idea on timeline for feedback here?