This fixes the app in firefox & firefox mobile

Apparently EmberLeafleet crashes if the center is not defined by the time the view is initialized, and in firefox the geolocation works in an asynchronous fashion.

Now the center is initialized to an arbitrary position, and updated with an observer (it used to be a computed property)
I grepped the codebase for that, and since it wasn't used I removed.

If you really miss it, I'll add it back.