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/test/util_test.lisp

@@ -0,0 +1,9 @@
+(in-package :panettone.tests)
+(declaim (optimize (safety 3)))
+
+(test add-missing-base64-padding-test
+  (is (string=
+       "abcdef"
+       (base64:base64-string-to-string
+        (panettone.util:add-missing-base64-padding
+         "YWJjZGVm")))))