Remove rel="" when inserting a media link in the WYSIWYG + allow for scalar:child_urn to be a page slug (in addition to a version URN)

This corrects a problem where:
1) User add a media link in the WYSIWYG
2) User makes a new version of the linked media page
... then the dcterms:references relationship is no longer valid
Now, users can make the incremental media page versions and the
referencing page will correctly maintain the reference relationship.
Merge branch 'master' of https://github.com/anvc/scalar