Maintain map states in hash for ProposalPage

[ericsoco]

Store zoom/lat/lon in the hash for every map (current, proposed, diff) that supports pan/zoom.

Thinking of use case like e.g. 2003 Texas Redistricting, in which a specific part of the overall district map (in TX 2003, Travis County) is worth zooming in on. Could be valuable to zoom into a place of interest to share / annotate, e.g. on social media, in proposal page comments, etc.