Persist Layergroup State

[chriswhong]

User Story

As a frequent ZoLa user, I want ZoLa to remember the layers I've toggled on/off between sessions so I can save time on my next session.

Description

Layer toggling is persisted in the URL parameters, but if landing on the default ZoLa url, it would be helpful if ZoLa remembered the layers that were toggled on/off from the previous session. We could use localstorage for something like this.

[chriswhong]

To reproduce:

• Turn on 3d Buildings (Map pitch changes automatically)
• Manually change the map pitch back to overhead view
• Toggle any other layer (Map pitch changes automatically, as it did when you turned on 3d buildings)