implement avatar backend functionality

go/migrations/000017_human_avatar.down.sql

@@ -0,0 +1,6 @@
+BEGIN;
+
+ALTER TABLE humans
+  DROP COLUMN IF EXISTS avatar_object_id,
+
+COMMIT;

go/migrations/000017_human_avatar.up.sql

@@ -0,0 +1,6 @@
+BEGIN;
+
+ALTER TABLE humans
+  ADD COLUMN IF NOT EXISTS avatar_object_id TEXT NULL;
+
+COMMIT;