PURPOSE
Reorganize website_forum module. This allows to better locate code, understand
organization, and prepare future changes.
SPECIFICATIONS
Split data files / model. It is easier to have a global view of a module
organization and content. It also allows to see technical data impacted
by the module installation (like ir_actions, mail_message_subtypes, ...)
Task-3240082 (Forum: Reorganize and lint module)
Part-of: odoo/odoo#114841
