Add ability to import video and slide URLs after the conference

[kmccurley]

Once the conference is over, there are sometimes videos and presentations available. Those eventually get indexed in cryptodb, and it's possible to pull them back into the program once you have the DOIs. The API for doing this is https://iacr.org/cryptodb/data/api/paper.php?doi=10.1007/978-3-319-96881-0_25

We should add this import capability.

[kmccurley]

It's easy to modify the editor to handle addition of video URLs, but there are two problems:

1. the screen for editing a talk needs to be two columns and compacted to fit on one screen.
2. there really should be a programmatic way for authors to upload their slides and be matched up to the id in the program. That would require a new webapp that allows authors to upload and "claim" ownership on the slides for a talk. It's a natural feature to extend websubrev. We could then pull this information to the program editor (or eventually unify the two tools, which are very different).

[kmccurley]

There is still a bug where the slides URL is not editable - the modal doesn't get populated when you edit a talk.