Add avatar field to Person and Group

I think we should add another image field to the Person model, to store an avatar. The avatar shall be the image used for public display, in contrast to the regular photo field, which shall be designated for internal use (of course, users can choose to use their official photo publicly as well).

At Teckids, we want to store an internal photo for official name lists (like those handed out to tutors to be able to identify the people in their workshops), and allow users to set a default picture shared with Matrix, Discourse, etc. as their public profil image.

While at it, we could add it for Group as well, to be used as chat room icon when managing Matrix rooms and the like