*: im_livechat.
Since 17.0, the new message separator is displayed for guests. The
`set_last_seen_message` rpc is also triggered in order to save this
information on the server side. However, the `_channel_seen` method
does not handle guests as it should.
As a consequence, the new message separator is always displayed and
the rpc call is always done when a guest loads the public page.
This PR enable guest support for this feature. Not that for live chat
channels, visitors cannot access the operators seen infos.
task-3603603
closesodoo/odoo#142546
Related: odoo/enterprise#52118
Signed-off-by: Sébastien Theys (seb) <seb@odoo.com>