Prompt a user to sign up for an API key if they use Mapzen tiles without one

[louh]

As of 7b3f0ea35725abbb2a91038543169fc4d8dc1759, Tangram Play has an internal flag (app.mapzenAPIKeyInjected) that is set to true if a scene file uses a Mapzen tile source without an API key (which causes Tangram Play to inject its own API key). We can subscribe to this flag being set to "true."

What do we do with this? One idea is to trigger something in the UI that prompts users that they need a Mapzen API key for their tile source. What does this look like and how does it work?

Related question: should this wait until we have access to a user's existing API keys? (which would also allow for injecting the user's own key.)

@souperneon