fix(web/panettone): Don't add extra padding when already padded

Because of math being upsetting, we were adding 4 padding characters to
an already-properly-padded base64 string, which broke tazjin.

This also breaks this function out into panettone.util, and adds a test
for it.

Change-Id: I7bc8a440ad9d0917272dd9f2e341081ea14693da
Reviewed-on: https://cl.tvl.fyi/c/depot/+/5782
Autosubmit: grfn <grfn@gws.fyi>
Reviewed-by: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI

web/panettone/src/packages.lisp

@@ -1,7 +1,7 @@
 (defpackage panettone.util
   (:use :cl :klatre)
   (:import-from :alexandria :when-let)
-  (:export :integer-env))
+  (:export :integer-env :add-missing-base64-padding))
 
 (defpackage panettone.css
   (:use :cl :lass)