[IMP] mail: reorganize mail message code

Purpose of this commit is to reorganize mail.message file in order to
better understand its content. It is done according to guidelines

  * computed fields;
  * CRUD and search / check access overrides;
  * actions (discuss, moderation, fetch / failure);
  * tools;

Even if this commit generates a big diff, there only some main blocks code
move. Nothing is changed and functionally as well as technically nothing
changes.

closes odoo/odoo#38695

Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>