Visibility page map enhancements

[dgboss]

Right now, the map on the visibility page uses the extent of the world when the page loads. This means that a very large amount of data gets requested and slows things down.

We can explore a couple of options for improving performance and the user experience:

1. Use geolocation to identify the users position and set the extent at a certain level (experiment to see what appears reasonable).
2. If navigating from the main map page, use the center point of the map from the main page as the center point of the map on this vis page and zoom to an appropriate scale.