Remote subscribe button for remote PeerTube accounts' playlist

[JohnXLivingston]

Hope this is not a duplicate.

When we are on an instance on which we have no account (for example when coming from a link in a social network), we have a button «subscribe» were we can enter our account and subscribe from a remote instance.
It will be very usefull to have to same feature for adding the video in our «see later» playlist on our remote instance.