base64 padding

The original base64 spec has pointless = trailers.

According to conventions.md

we sometimes omit trailing = padding characters

But typically, when base64 is referenced in the wider spec, the presence or absence of padding is not stated.

• Every place we reference base64 should answer this question
• References to base64 encoding shouldn't appear ad hoc eg in individual netdoc item descriptions. The encoding should be stated centrally where the type being encoded is defined. (This means that the same type should always be encoded the same way, which may not currently be the case.)
• Probably, all this will involve giving names to the with-padding and without-padding encodings.
• New protocol elements should probably omit the padding since it is useless.