WIP: Path to locality- & date-nested contributions data in committee.html

[ckingbailey]

HEADS UP: This work depends on yet-to-be-merged backend changes.

This work resolves #435.

Updates committee.html to get the contributions data for the committee at the path _data/committees/<locality>/<election-date>/<committee-id>.json, or in Jekyll-ish terms, site.data.committees[locality.slug][election_day][committee.filer_id].

Adds Jekyll "where" queries to get locality and election date for committee from site.localities and site.elections. Is there a better way to do this, perhaps building that information right into the committee's markdown file?

:shrug: NOTE: The query to get election date depends on committee.election, which is not a key that exists on the committee markdown file, and yet it seems to be working by some mystical interdimensional force. Maybe we should understanding how this is working before this gets merged.

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