Show outline of feature that is currently being hovered over

[dellisd]

When a feature is moused over, an outline of that feature should be highlighted.

This can be done using mapbox's "feature state" functionality, which allows the styling of a feature to be adjusted if a feature is selected. Alternatively (and necessary if we only want to show the outline), the feature that is being hovered over can be copied to a special line layer that is styled to look like an outline.