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
Hiral Bhavsar
b2de618f9f