We want to drop the generic #avatar_icon helper that supports both an email
and a user object being passed in. Instead, we want to explicitly use the #avatar_icon_for_user
and #avatar_icon_for_email helpers depending on what we have at hand.
This allows us to avoid unnecessary database queries (e.g. call
User.find_by_any_email if we already have the user).

In situations like here, this makes it less convenient to use.

Removes 'hidden-xs' class from user_avatar helper in order for it to be visible in environments list.
Allows a class as an option to keep the hidden avatar behaviour in all the places it is needed.

Closes #21216