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/default.nix

@@ -42,6 +42,7 @@ depot.nix.buildLisp.program {
       ./test/package.lisp
       ./test/model_test.lisp
       ./test/inline-markdown_test.lisp
+      ./test/util_test.lisp
     ];
 
     expression = "(fiveam:run!)";