User Geolocation [Merge with caution]

[jthiller]

Some tasks:

• [x] Persist user zoom level when auto following.
• [ ] Style geolocate button
• [ ] Keep button within viewport on mobile screens
• [ ] Determine way to let user know that the browser does not have location permissions
• [ ] Keep screen from dimming when auto following.
• [ ] Make sure tiles are rendering (https://github.com/helium/mappers/issues/73)