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/util.lisp

@@ -5,3 +5,11 @@
    (when-let ((str (uiop:getenvp var)))
      (try-parse-integer str))
    default))
+
+(defun add-missing-base64-padding (s)
+  "Add any missing padding characters to the (un-padded) base64 string `S', such
+that it can be successfully decoded by the `BASE64' package"
+  ;; I apologize
+  (let* ((needed-padding (mod (length s) 4))
+         (pad-chars (if (zerop needed-padding) 0 (- 4 needed-padding))))
+    (format nil "~A~v@{~A~:*~}" s pad-chars "=")))