After removing the `sha_in` params a while ago, we get rid of
the deprecated `token_field` option.
- Make `token_field` a model attribute, so that each model can easily
define the token field that should be used, and it does not need
to be passed around all the time anymore.
- Rename `_special_access_object()` to `_has_token_access()`, much more
readable since it returns a bool
- Do not forward the `attachment_ids` keyword arg to message_post,
as it sometimes contains unrelated IDs (the helper is not meant
to post attachments anyway)
- Update callers accordingly
Olivier Dony