Add Embedded Social to Nearby (map) view

[barbeau]

Summary:

An interesting app called "Pigeon" surfaced recently, and it shows social posts in context of transit for NYC (still invite-only for iOS):
https://www.theverge.com/2018/5/31/17414330/google-area-120-pigeon-nyc-subway-app

They have an interesting map view that shows social posts as pigeon markers:

We could do something similar with Embedded Social on OBA, such as showing red dots on hot stops, or after we have user-generated topics the topics themselves could be geo-tagged with their own icon.

Right now this is blocked by a working "Popular" topics implementation for stop and route-specific topics.

Steps to reproduce:

1. Go to Nearby view

Expected behavior:

Show me something that indicates where hot stops/routes/topics are located

Observed behavior:

We don't show any Embedded Social info in context of the map