Support ImageProperty and VideoProperty properties using forms

[benfrancis]

For W3C compliance Thing Descriptions now use forms instead of links for properties, actions and events.

Historically, instances of ImageProperty and VideoProperty would provide a link with rel: 'alternate' and mediaType: 'image/jpg' which a client would use to fetch binary files to display on the screen.

The front end may need updating to use forms instead of links to fetch these resources. Currently the Virtual Things Adapter is still providing links, but I think displaying them may be broken on master. Other add-ons may do the same.

See also:

• Change links to forms in Camera and VideoCamera virtual-things-adapter#68
• meta operations when multiple content types exist w3c/wot-thing-description#1814