Add "emoji whispers" mode

[mhthies]

Add a game mode, where text messages and emoji-only messages are passed on alternately, instead of questions and answers.

This would only require modified messages as well as a modification of the game logics, such that a page usually has an odd number of entries and starts and ends with a text message (instead of starting with a question and ending with an answer).

We could also constrain the character set of the emoji messages to refuse alphabetic characters.