Change default / contacts text ringtone

It would be really useful to change the default text ringtone. As well as a different ringtone for different people.

No files, tests, or entry points are named. Start by locating the iOS notification sound settings and the code that handles contacts, then determine how default and per-person text ringtones are configured; done means both options work reliably and are covered by the project's relevant tests.

Written by the indexing model from the issue text.