Recurring invoices generated from contracts now prepare invoice line details through a separate reusable step. This makes it easier for custom implementations to adjust invoice line values without changing the main invoicing flow.

In recurring invoices from contracts.
It allows to inherit and modify the prepared values for the lines.

Details about the reason of the change on odoo/odoo#6822

This update prepares Odoo's messaging area for future Slack-style chat changes by renaming group-related pieces to channels and tidying related mail data. It also removes old employee and group suggestion features and simplifies some unread message handling, with limited direct business impact now but smoother future collaboration improvements.

This PR contains all commits before the slack refactoring itself.