[FIX] mail: fix tags disappearing on message sent

This commit is a small fix to avoid reloading several form views
(like crm leads or helpdesk tickets) when a message is posted in
the chatter. This is linked to an old issue where relational fields
like the tags do not keep changes properly in the base model,
therefore dropping the changes on reload in this case. Do note that
it will still reload (and keep the bug) when a recipient is linked
to the message and perform the initial purpose of the reload (like
updating the customer field) as it was done before.

opw-3245717

closes odoo/odoo#117500

Signed-off-by: Aaron Bohy (aab) <aab@odoo.com>