[IMP] im_livechat, website_livechat : add livechat username

To avoid that people know the complete name of the support members
(for privacy purpose), a username can be used.
Users have the opportunity to set and modify their username in
their user preferences.

If the username is not set, the complete name will still be used.

If the username is set, it is used for the discussion title, the
name of the message sender, the name of the team members in the
channel statistics. All of this, only in the livechat context.

Task ID : 1895998
Closes PR  #28283

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