GroupChat, take 2

Following our extended discussion on slack yesterday about how to model a chat-room in DAML, and some of the issues this raises, I persevered with my own model, and then studied the models linked to by Bernhard and Ognjen.

In the end, my model looked rather similar to that of Ognjen's, whereas Bernhard's is quite a bit more complicated, I have a feeling this is because Bernhard is modelling additional features over and above the simple WhatsApp style functionality I have in mind. But we need to discuss more so I really understand.

This PR has the updated & simplified version of my GroupChat model, together with testing scenarios.
There is an extensive README which describes the Domain features being modelled, and discusses alternatives.

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