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.
Does `idOrEmails.length` refer to the number of other people in the conversation, excluding oneself? If so, this looks work. Otherwise I think we may need to modify this to `if (idOrEmails.length < 2 || idOrEmails.length >  6){`.