Before this commit, when a user creates a private channel without adding himself
as a member of it, it raises an access error. After this commit, user will be
automatically added as a member by default when creating a channel.

It makes sense the creator is a member of its channel. Moreover it avoids
issues on private channels.

Task 2172420
PR odoo/odoo#44454

Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>