Embed videos on paper page

It would be nice to embed videos on the paper pages. This shouldn't be hard to implement. It would require:

1. Modifying the paper page template, `hugo/layouts/papers/single.html`, to embed the video
2. Maybe writing a custom field in the generated YAML that is used by Hugo to build the Anthology. This would be done by creating a field, say `video_embed_link`, in the `from_xml()` function in `bin/anthology/papers.py`. (This file is used to load papers from the XML file, and all of its `attrib` attributes are dumped to the YAML).

We link to videos on Vimeo and Slideslives, which likely have different embedding code blocks, which is the reason for point 2 above.

Start with hugo/layouts/papers/single.html and bin/anthology/papers.py, especially from_xml(), which the issue says feeds attributes into generated YAML. Check how paper metadata is currently exposed to Hugo and how video links for Vimeo and Slideslives are represented. Done means paper pages can render an embedded video when the new metadata field is present.

Vom Indexierungsmodell aus dem Issue-Text verfasst.