Unify treatment of "external" URLs

[trieloff]

@asteed's namespace sweep (#179) has produced a couple of interesting findings:

On the one hand:

https://ns.adobe.com/external/angellist
https://ns.adobe.com/external/deviceatlas
https://ns.adobe.com/external/dnb
https://ns.adobe.com/external/isic4
https://ns.adobe.com/external/naics
https://ns.adobe.com/external/sic

on the other hand:

https://ns.adobe.com/xdm/external/activity-streams-2/activity
https://ns.adobe.com/xdm/external/activity-streams-2/collection
https://ns.adobe.com/xdm/external/activity-streams-2/collection-page
https://ns.adobe.com/xdm/external/activity-streams-2/context
https://ns.adobe.com/xdm/external/activity-streams-2/context#/definitions/expanded-term-definition
https://ns.adobe.com/xdm/external/activity-streams-2/context#/definitions/single-context
https://ns.adobe.com/xdm/external/activity-streams-2/document
https://ns.adobe.com/xdm/external/activity-streams-2/id
https://ns.adobe.com/xdm/external/activity-streams-2/image
https://ns.adobe.com/xdm/external/activity-streams-2/link
https://ns.adobe.com/xdm/external/activity-streams-2/object
https://ns.adobe.com/xdm/external/activity-streams-2/object#/definitions/mime-media-type
https://ns.adobe.com/xdm/external/activity-streams-2/rdf-langstring
https://ns.adobe.com/xdm/external/activity-streams-2/rdf-langstring#/definitions/tag
https://ns.adobe.com/xdm/external/activity-streams-2/type
https://ns.adobe.com/xdm/external/activity-streams-2/type#/definitions/type
https://ns.adobe.com/xdm/external/hal/link
https://ns.adobe.com/xdm/external/hal/resource
https://ns.adobe.com/xdm/external/hal/resource#/definitions/hal
https://ns.adobe.com/xdm/external/repo/accesscontrolentry
https://ns.adobe.com/xdm/external/repo/accesscontrolentry#/definitions/acl
https://ns.adobe.com/xdm/external/repo/accesscontrolpolicy

I'd probably go with the second variant for consistency.

[asteed]

@trieloff can this issue be considered closed?