Add ability to download a video from direct link or torrent file

...src/app/videos/+video-watch/video-download.component.html

...t/src/app/videos/+video-watch/video-download.component.ts

...nt/src/app/videos/+video-watch/video-watch.component.html

client/src/app/videos/+video-watch/video-watch.component.ts

client/src/app/videos/+video-watch/video-watch.module.ts

client/src/assets/player/peertube-videojs-plugin.ts

server.ts

server/models/video/video-interface.ts

server/models/video/video.ts

server/tests/api/multiple-pods.ts

server/tests/api/single-pod.ts

shared/models/videos/video.model.ts