feat: Add profile pictures to OAuth users (#3855)

This supports GitHub and OIDC login for profile pictures!

coderd/database/migrations/000044_user_avatars.down.sql

@@ -0,0 +1,2 @@
+ALTER TABLE users
+    DROP COLUMN avatar_url;

coderd/database/migrations/000044_user_avatars.up.sql

@@ -0,0 +1,2 @@
+ALTER TABLE users
+    ADD COLUMN avatar_url varchar(64);