Converting to Kotlin

[cguess]

I just received a Mozilla Open Source Software grant to convert Android to Kotlin. This is the issue that will track that progress.

However, what we'll also do is start small, write tests for a class, convert to Kotlin as the tests pass. Changing the architecture wildly is probably not a good idea at this point.

The android app needs more love than iOS so some refactoring may be necessary. If so, tests should still be written, but commit messages have to be very explicit.