For #296, we'll add a simple "person"-icon placeholder for an avatar image, to use when the user is muted.

Perhaps we could reuse this for when the real avatar doesn't load, either because the URL is invalid or, with RealmContentNetworkImage.errorBuilder, because its request fails (an instance of #265). Or, pre-#1556, for a message from an unknown sender (but we should eventually do #1556 for that).

This won't be ripe to work on until #296 is resolved. Please don't try to claim this issue if #296 is still open.