This change continues the modernization of Odoo's public livechat experience by aligning it with the newer Discuss architecture. It should make future livechat improvements easier to deliver while keeping the customer-facing chat flow more consistent and maintainable.

**/!\ EXPERIMENTAL PR /!\**

*: website_livechat

This commit is a step towards refactoring the JS of public livechat,
so that it reuses the same architecture as the code of Discuss.
This implies code that uses JS models and OWL components.

Task-2928837

This update refreshes underlying discussion and activity-related code while also improving calendar, CRM, and website behavior. Business users should see a more reliable communication experience, with fewer glitches around destroyed UI elements and activity menus, while most changes are internal preparation for future improvements.