Add config for prohibited characters in username.

[digisomni]

vircadia/vircadia-native-core#1457

We should determine if the Metaverse is unable to work with certain characters in the username to begin with. If that is so, we should try to solve that. If this is an issue in the Domain servers, we should solve the issue there.

Then, we should still add the configuration for prohibited characters in a username (if it does not already exist?) irrespective of where the issue is.