add canonical template to see landmark ordering

[jabooth]

The central idea here is to help users convey how a landmark set should be used. Right now templates encode the rules of the number of points that should be placed and the connectivity between them, but a user cannot be given a template in isolation and know how to do the landmarking.

One idea here would be to allow users to embed URLs in templates to helper material on how to perform some landmarking. I'm imaging here that a template-writer could simply make a YouTube screencast explaining how to do landmarking with this template. This could be as in depth or quick as you want.

It could be that simply the landmarker redirects to youtube for you to watch the video, or it could be that we embed the youtube video in the landmarker UI somewhere.