Don't let the user scroll off of the map

[orblivion]

Reported by @Ryuno-Ki

If you zoom out all the way and start scrolling, it's possible to lose the world map entirely.

Suggested approaches:

• "Reset" button for zoom and location
• Looping map
• Edges on map

I suspect the third option is the simplest; there's probably something built into Leaflet. I did want something like the first one at some point regardless, but I don't want to take the time for launch. I'll split that off.

[orblivion]

appVersion 35 (new app-id: https://apps.sandstorm.io/app/e5eaqnrqfrhgax1awtgw9uqayg42kcen2gkpynjs3j5mww7w3rp0?experimental=true)