[IMP] mail: add Many2OneAvatarUser field widget

This widget is an extension of Many2OneAvatar, designed for
many2one fields pointing to the 'res.users' model, or a model having
itself a many2one field pointing to 'res.partner'.

With this widget, when the avatar is clicked, we open a DM chat
window with the corresponding partner. If the user clicked on itself,
we open a blank chat window for the sake of consistency.

In kanban views, this widget only displays the avatar.

Part of task 2195254