Warn user of duplicate unregistered names

[holmesworcester]

As part of #1340 we are letting unregistered users send messages. We should use these designs to warn users if an unregistered user has the same name as a registered user or as another unregistered user:

In the message screen, all duplicate usernames should have a "duplicate" warning:

https://www.figma.com/file/TV9pF84Ob8pLYRLu83gNol/Joining-when-owner-is-offline?type=design&node-id=311-7523&mode=design&t=eS9WDLYpQNod1Acl-4

Tapping on this warning should take users to an explanation and warning:

https://www.figma.com/file/TV9pF84Ob8pLYRLu83gNol/Joining-when-owner-is-offline?type=design&node-id=320-2662&mode=design&t=eS9WDLYpQNod1Acl-4

[kingalg]

@holmesworcester this issue is about unregistered user having the same username as registered user. Is it also possible that two unregistered users will have the same names? I can't find issue for this and I was thinking if this is technically possible or not?

[holmesworcester]

Multiple unregistered users having the same name can be handled with the same warning as registered/unregistered. This issue covers both cases.

[kingalg]

[email protected]

Done on Desktop and Mobile. All issues discovered in this area has been reported in separate github issues.