feat(bot): allow separate member avatars for proxied messages (#523)

This allows for using one avatar for the member card, and a different
avatar for proxied messages - so that users can set the main avatar to
a "full" version of their avatar, and the "proxy" avatar to a cropped
version.

PluralKit.Core/Database/Migrations/33.sql

@@ -0,0 +1,6 @@
+-- database version 33
+-- add webhook_avatar_url to system members
+
+alter table members add column webhook_avatar_url text;
+
+update info set schema_version = 33;