Disable elastic page scrolling on Safari and mobile Safari

[quincylvania]

Safari does this as a nice way to indicate you've reached the end of the page, but it doesn't make sense for an app like iD that takes over the whole window. On desktop it's no big deal but on touchscreens it gets annoying.

[quincylvania]

I tried a lot of different ways to do this and found that it's pretty complicated to disable page scroll bouncing without also disabling scrolling for all elements within iD. I implemented a basic solution that only works for standalone iD (e.g. on the preview site but not on osm.org). To me it makes iD feel less like a webpage and more like a real app, which is great.