Further positioning fixes

[antony]

• [x] On a very small viewport, make the calendar fullscreen and scrollable
• [x] Ensure that the time chooser part is no bigger than the overall calendar
• [x] Make sure button doesn't appear within no-click zone on stupid iPhone
• [x] On a wide but thin screen, show the side-by-side range picker

[antony]

There will be edge cases to this, so leaving here to clean-up. However I think I've got positioning into a working state for now, across a variety of devices and iPhone and iframe.

Remaining tasks are:

• [ ] Clean up positioning code
• [ ] Tests for positioning code (it was trial-and-error, so now I've solidifed it we can go back and add testing)
• [ ] Turn positioning into configuration, and pass it in so we factor in the calendar-width attribute
• [ ] Decide on a "mobile" size, currently we have a mix of 480, 600, and 680
• [ ] Tidy up the fullscreen view by adding some padding on small screens