Improves on emoji picker / message box

[diegolmello]

• Emoji picker should be opened "behind" native keyboard (Custom Keyboard)
• Message box shouldn't use setState
• Interactive dismiss of keyboard using swipe down (iOS)
• Use FlatList on Emoji picker
• Check mentions and autocomplete

This lib could help: https://github.com/wix/react-native-keyboard-input