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.
closesodoo/odoo#38695
Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>