Hover effects

[wmerrow]

At the national view when hovering over a state:

• tooltip following cursor (currently there, doesn't need changes)
• dark gray stroke around the hovered state
• pointer cursor to indicate clickable

At the state view when hovering over a tract:

• tooltip fixed at right of screen (currently there, doesn't need changes)
• dark gray stroke around the hovered state

[bsouthga]

@wmerrow I've added a basic hover effect -- however there are some performance issues (internal to mapbox: mapbox/mapbox-gl-js#2874) that I will need to spend some time on to make it look better (and make the tract hover usable at all), I probably won't have time to do this tonight.

[wmerrow]

@bsouthga let me know if you think of a different hover effect that would be easier to implement instead of a gray stroke, although I think that would be the best solution from a design perspective.

[bsouthga]

@wmerrow yeah I agree, the issue is pretty general to hovering in mapbox-gl maps with large numbers of polygons (in this case 30k tracts). It looks like there is a possible solution, just need to spend some time tonight investigating.

[bsouthga]

(note for self): mapbox/mapbox-gl-js#5040