Multi is the default api for methods, it is not necessary to explicitly
decorate methods with it, adds clutter and most people use it because
they see that the rest of the code uses it.
Done with `find . -type f -name '*.py' | xargs sed -i '/@api.multi/d'`
Purpose of this commit is to clean notification process: calls, methods
API, method name, variable propagation.
Contains notably
* simplify API of methods used to group recipients when sending notification
emails;
* improve and rename methods used in email notification process;
* move some methods on model itself as non mail thread records could be
mass-mailed and _notify_email_headers could be called on other records;
Related to task 1943901
Linked to PR #32404
The activity view has been greatly improved to allow to customize it
more easily. It works quite similarly to the kanban view, defining
`<field>` tags at the top and using these fields in the `<template>`
section. The template name used to define the activity cards is
`activity-box`.
Note that these activity cards are rendered using `KanbanRecord` widget
(this has implied that ActivityView inherits from `BasicView`).
Also note that the view validation has been moved in base to include the
common grammar.
Task 1894990
Adrian Torres