[message-list] Move avatar in vaadin-message to a slot

[web-padawan]

Describe your motivation

The vaadin-message-avatar element in the vaadin-message component is currently in the shadow dom.
This means that all the shadow parts in vaadin-avatar would require using exportparts attribute.

Describe the solution you'd like

Move the avatar element into a slot in vaadin-message, change from vaadin-message-avatar to vaadin-avatar.

[web-padawan]

Fixed by #4702