Human-oriented links to maps

[davenquinn]

We've recently inaugurated a slug concept for Macrostrat maps. This is similar to the map_id but is geared towards human-readable use.

Currently, public-facing map URLs use the map_id as the primary representation: https://v2.macrostrat.org/maps/260. But maybe we should change this to https://v2.macrostrat.org/maps/gmna/ etc. Or maybe even (optionally) put an organization page in front of it for different map providers?

This is all part of making Macrostrat a better platform for interacting with individual maps (which is useful for broader provider buy-in, credit sharing, etc.)