We'll need to introduce TextInputs for username and password selection. This diff extracts the TextInput code out of KeyserverSelection so it can be reused.

Play around with the KeyserverSelection screen and make sure things work as expect:

1. Try focusing / bluring it
2. Make sure we're able to programmatically focus / blur it
3. Make sure it still looks the same
4. Make sure the style on focus still looks the same