[FIX] mail: avoid to post multiple join message on channel
[FIX] mail: avoid to post multiple join message on channel

When a new user is subscribed to a channel, (multi-user, without emails)
a message is posted on this channel.

The mechanic used for that is unfortunately not very reliable
Javascript will call channel_join_and_get_info every time a channel
is added (client side) even if the user is already in the channel.

This fix simply check that the action_follow actually add the
user to the channel before sending the message.

We should avoid to call channel_join_and_get_info in this case and only call
channel_info but this fix will temporary fix the issue.

closes odoo/odoo#29441
ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo