[FIX] mail: visitor to portal_user should keep livechat channel

adapting #20279 to v10

Before this commit, when starting a livechat session as a public user and continuing as a portal user, the livechat crashed
(the poral user couldn't see the channel created before as Admin)

After this commit, the conversation is kept and security insured by the uuid in the cookie

OPW 775568

closes #20281