Conversation name uniqueness

[KevinWorkman]

https://github.com/Priya411/CodeUGroup3/blob/master/src/codeu/chat/client/core/UserContext.java#L64

There is logic for making sure a user name is unique, but I don't think there's logic for making sure a conversation name is unique? This will cause problems if users create conversations with the same name. This loop will only ever find the first one.

Probably just need to add the same check that was already added for user names.