Embedded media like images and video can now individually be collapsed

A new preference "Collapse media by default" is also added, the default
is to not collapse media.

Since images are not always shown, "showImage" is also renamed to
"loadImage"

Closes #1187

Enhance Spotify transformer to handle syntax with 4 groups like `spotify:user:spotifydiscover:playlist:2RjlnjirDnFioPelxk0g4M`

Encapsulate spotify integration inside rocketchat:spotify package. No more inside rocketchat:oembed.

Remove rocketchat:spotify from default list of packages inside `.meteor/{packages,version}`

- https://open.spotify.com/track/34AWo71Ya5gq7wpNnatwr7
- spotify:track:34AWo71Ya5gq7wpNnatwr7

Works with tracks, albums, artists and playlists.