Add links and attachments to talk details

[cko]

the frab export (https://schedule.nixcon2017.org/en/nixcon2017/public/schedule.json) contains fields for links and attachments (e.g. slides of the talk). Both should be added to the talk details.

[maxhbr]

These fields are currently not filled and the underlying datastructure is unclear. Will be implemented as soon as example data as present.

[maxhbr]

Links are filled for at least one talk:

raw:

"links":[{"url":"https://bazel.build","title":"Bazel"},{"url":"https://gittup.org/tup/","title":"Tup"},{"url":"https://github.com/edolstra/nix-make","title":"Nix-Make"}],"attachments":[]

[cko]

As frab is a widely used conference management system, you can just have a look at another conference eg. FrOSCon or 33C3