Enforce chat name uniqueness

[rowanseymour]

Enforcing uniqueness for contacts has to be handled on the RapidPro side. ChatPro should enforce uniqueness between users, and could prevent a user from taking a chat name used by a contact.

However we can't currently prevent RapidPro from giving a contact a chat name taken by a user. Administrator users are also not tied a single organization so it might not be feasible to prevent them from taking chat names already used within an org.