Before this commit, when editing a message, it was not possible
to add new mentions.
This commit adds the support to add new mentions when editing the
message.
Note that mentions added from editing message won't notify them:
it's only at creation of message that mentioned users are notified.
task-3329714
closesodoo/odoo#126888
X-original-commit: 1fe935d273142064784b9ecff2fff1c34e91064f
Signed-off-by: Alexandre Kühn (aku) <aku@odoo.com>