Add thumbnails for Child Talks via ThumbnailExtractor and Downloader

[marcoroth]

This pull request adds a Talk::ThumbnailExtractor and Talk::Downloader which allows a meta talk YouTube video to be downloaded so we can extract thumbnails for each of the segments for the child_talks of a Meta Talk.

The video only has to be downloaded once so the person who is committing the new talks can run the rails download_missing_meta_talks rake task to download the videos for which the thumbnails are missing and then run the rails extract_thumbnails rake task to actually extract the thumbnails.

Now the CI (seed_smoke_test) will fail if somebody forgets to extract the thumbnails, since we now run the rails verify_thumbnails rake task in CI to make sure we always have all required thumbnails.

[adrienpoly]

Sweet !