[REF] {website_}mail: move and replace published field of mail.message to is_internal

PURPOSE

Clean posting process and improve mail.message definition and comprehension.

SPECIFICATIONS

Website mail defines a website_published field allowing to publish / unpublish
comments on the frontend of some modules. This field has several drawbacks :

  * it is used only for front-end people (portal, public) and has no real
    effect in chatter / classic discussions;
  * it is used only in some advanced front-end module and is not available
    in portal by default;
  * its naming is not really correct as it is not linked to fields coming
    from the website_published mixin and its behavior is not really
    the same;
  * its use is a bit duplicated with internal flag coming from subtype
    allowing to hide messages related to an internal subtype;
  * there are overrides of standard mail.message methods just to handle
    this flag;

In this commit we change that field by an is_internal flag directly on
mail.message model itself. It tells if share people (customers, share users)
are allowed to read the message. This field can be given through posting
API or set manually using widgets. It is also used in access rights custom
methods and managed like the internal flag of subtypes.

Mailgateway was already using an internal flag for internal note replies. It
is renamed to is_internal and propagated as it is now a standard field. It
also eases code understanding.

Portal is updated to allow managing the flag directly. It means customer portal
now natively allows to moderate customer comments without any need of website
modules.

Rating is updated accordingly. An is_internal field is added, replacing the
related on website published.

LINKS

Task ID 2071556
PR #38692