Webhook avatars rewrite

There is another drawback. Converting from PNG to WEBP or some other format is basically not possible for now unless we add something to mediaproxy to do it itself, and while that's feasible, I'm not up for it as long as the official client is happy with it (and it looks like so).

Also this fixes ./manage.py migrate since this has the first migration post-the-whole-migration-table-change we had some time ago.