Updates to custom models and fields are now handled more safely and consistently, reducing the chance of partial changes that can leave databases in an inconsistent state. The registry and cache refresh process was streamlined so changes are applied transactionally at the end of a request, making customizations easier to extend and maintain.

Refactor the code, and improve it in the following ways:
- remove deprecated class `RegistryManager`;
- simplify APIs of registry setup by removing parameter `partial`;
- delegate addition of custom models/fields to models `ir.model` and `ir.model.fields`, which makes it easily extensible;
- always perform registry/cache signaling at the end of request, and make updates on custom models/fields fully transactional.

This update makes timesheet entry and approval smoother by improving how grid edits are handled, separating employee and manager workflows, and assigning clear timesheet validators. It also adds automated reminders so employees and managers are prompted to fill in or validate timesheets on schedule.

Sub of https://github.com/odoo/enterprise/pull/788/commits

Task: https://www.odoo.com/web#id=29568&view_type=form&model=project.task&action=333&active_id=965&menu_id=4720