Don’t ask for video’s tags again if video has tags

Because of the previous commit, retrieving a collection of videos
might actually return tags, if the collection was authenticated and
the user has permission to see the tags.

Therefore, if a snippet already contains tags, don’t erase them and
load them again: just show the ones already loaded.
Don't read nested videoId from videos#list
Merge pull request #83 from Fullscreen/fix-video-list

Fix video list