Notes from conversation with Jonny:

If we're on iOS, we should aim to open the .hex in the micro:bit app.

It seems that there is no way to determine whether an iOS device has an app installed from the browser. This is a possible work around:
https://stackoverflow.com/questions/13044805/how-can-i-check-if-an-app-is-installed-from-a-web-page-on-an-iphone

The plan is to raise a new issue in microbit-ios repo, linking this branch and tagging Martin. We'd need to tweak the app so that Python .hex files are opened in the Python Editor instead of MakeCode if the user attempts to edit the project from the app. To do this we'd need to update the migration code to handle the current format, and a multiple project format. Perhaps we should do this first before raising an issue?

Current migration format as JSON:

{
  "meta": {
    "cloudId": "microbit.org",
    "comment": "",
    "editor": "python",
    "name": "beating-heart",
  },
  "source": "code as text"
};