Currently both incoming and outgoing emails are using the same 'email'
message_type. However both flows are not linked in any way.
Purpose of 'message_type' is to distinguish who generated the message.
In this case incoming emails are generated by the mailgateway while outgoing
emails are generated by mailins e.g. using the composer in mailing mode.
We now distinguish outgoing emails from incoming emails by using a specific
type for outgoing emails. Addons are updated accordingly.
Default 'message_type' value when removing sms/snailmail/whatsapp is now
'comment' instead of 'email', as default value of messages should be
comment as discuss is the main source of messages.
Task-3285720
closesodoo/odoo#139814
Related: odoo/enterprise#49597
Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>