Implement chat search

[saibotma]

chat-server does not manage user data, because this should be done by the application that controls chat-server and feeds it with data.

To support chat search chat-server needs to have some user data though, for example:

• first name
• surname
• username
• display name

This data will only be used for the chat feature, but never be given to any clients.
Users of the client API should not be able to alter this data, but only users of the platform API.