Add `slides_url` to `Talk` model

[marcoroth]

I noticed that some of the talks have an URL to the slides in the description. Maybe we can add that to the Talk model and link/embed them in a nice way next to the video.

For the Ruby Unconf talks (#114) I added a slides_url field to the YAML files where I had it available.

[adrienpoly]

yes we can try to add this field. I am afraid the links might get broken at some point On the other side I would like to avoid hosting files if possible so a link is the best solution for now

[marcoroth]

I think we could embed it, if we would whitelist the common slide hosters, like speakerdeck.com, or just embed it if it's just a plain PDF file.