PURPOSE
Reorganize addon according to guidelines, helping finding and updating code.
SPECIFICATIONS
Correctly split data into separate files. A lot of content was put a bit
randomly in data files, leading to a hard discovery of code and features.
We also fix duplicated attachments definition, both in mass_mailing_data and
ir_attachment files. It has been merged into the attachment data file.
Task-2864264 (Mass Mailing Module Reorganization)
Part-of: odoo/odoo#92509
