Enforce creation limits: names > 2 letters, convos 2-6 ppl

This PR enforces some creation limits.  It checks that names of new users are at least 2 letters long and no more than 128 letters.  It also ensures that conversations are created with only 2-6 people.
Great question Nikil!  Whether oneself is included in the list would definitely affect the logic.  As such, there should be some extra validation to check whether or not the user's self is included.  I'll revisit this code!