Since visitor banner has been added to the chat window (in discuss app),
the channel messages div has been pushed down (padding-top) to display visitor
banner without overlapping the channel content.
But this padding was always applied, even if there was no visitor banner
(in the case of normal -non live- chat). This commit only applies this padding
when the visitor banner is present.
Task ID: 2145749
closesodoo/odoo#41590
X-original-commit: 04f67217c4063d69477005a165b68b5ebecef01d
Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>