Add rate prompt

When the app is more mature. We still have things to improve before I feel comfortable asking for ratings.

We should be smart about when to ask. Maybe after the user has listened to 10 songs and the app is focused.

We can reuse this method: https://github.com/sindresorhus/Gifski/blob/cf635ab976d2b3291de9934f67876cb3b609d683/Gifski/ConversionCompletedViewController.swift#L76