Add a speedometer widget

This should display the user's current speed (which we should be getting from GPS), when it is available.

We had a prospective user say this was a hard requirement for them, and they are migrating from some archaic Mapbox SDKs. I have not personally used this, so I don't have a good intuition for what it should look like and how it interacts with the speed _limit_ view that we already have.

I'll push for some better definition, but in the meantime, perhaps @Archdoog or @haysmike have some opinions / screenshots / mockups to share (be sure to note source and license if it's not a permissive one so we don't accidentally run into issues).