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