Emoji

This MR introduces a custom Emoji implementation to Briar for devices that do not support Emoji sufficiently. It is heavily based on Signal's implementation. Hence, the license for the Android part has been changed to GPLv3.

So far, emoji input is only supported for forums and private conversations that both rely on the same `TextInputView`.

![emoji2](/uploads/f69af031e72efb0bd6f0f67a9574d11f/emoji2.gif)

See merge request !317