opus draft: Add missing date tags to references.

The xml2rfc DTD requires a <date> tag within reference > front.

Where the reference was to a published article, I tried to add
the correct publication date. For others, like references to
wikipedia, I just added an empty tag, since at this point
there's no way to recover a retrieval date.

With this change the draft is considered valid by xml2rfc 2.43.0.

However, it fails to include the externally generated text
for opus_source.base64.