[19.0][FIX] impersonate_login: stamp chatter messages on create

[astirpe]

mail.message.body and impersonated_author_id are stored computed fields
depending on author_id, so any later write on author_id re-runs them
outside the context in which the message was written. The stamp is then
lost, or the "Logged in as ..." notice is added a second time.

Both values are now set in create(), which also removes the body field
override and a compute that fired on every mail.message in the database.

Steps to reproduce (stamp erased):

1. As admin, Switch Login to another user.
2. Post a message in the chatter of any contact. The body starts with
   "Logged in as " and impersonated_author_id is the admin partner.
3. Go back to the original user and change the author of that message,
   for instance by merging the author partner into another contact.
4. impersonated_author_id is now empty, while the notice stays in the
   body: the record no longer says who was really behind the keyboard.

[OCA-git-bot]

Hi @Kev-Roche,
some modules you are maintaining are being modified, check this out!