Add an onboarding experience to explain the app

[pserwylo]

Yes, the best apps don't need explaining. This app will work just fine without explaining. However once we add the ability to record your own sounds (see #4), then it may be nice to at least explain that this is a possibility, so that people don't just always stick with the default ones. It may also help to explain things like screen locking (see #5).

Libraries such as https://github.com/AppIntro/AppIntro look like a great way to do this in an ergonomic and not particularly difficult way.

[pserwylo]

Now that contacts have been added, I don't think we need onboarding. Rather, we should add a "+" image alongside the existing contacts to add a new one, and put it behind the same anti-baby-tampering code that the screen lock uses (i.e. "Tap 4 more times to add a new contact").