Add changeset download link to save screen

[bhousel]

from #2739 (comment)

@bhousel is there any way to get the changeset data out of iD so it can be preserved and edited then uploaded (perhaps in JOSM)?

We already have this on the conflict resolution screen, so it should be easy enough to add to the general save screen in an unobtrusive way. It's very rare that someone can't save, but they shouldn't lose their edits because of an iD bug, and might help us in debugging the issue.

[bhousel]

We already have this on the conflict resolution screen, so it should be easy enough to add to the general save screen in an unobtrusive way.

Well this is interesting.. The code to open the changeset in a new tab no longer works because Chrome (and maybe others?) won't navigate to top level data: urls anymore 😢

https://www.chromestatus.com/feature/5669602927312896