Add web features to Digital Labels interface

[richbs]

There may be some iPads circulating at the opening of the Furniture Gallery. Would be great to have the following working by the end of November:

• Digital Labels index page shows a list of portals and a list of digital labels in a nice touchy interface
• Gateway object or biography image is shown on index page for each label/portal
• Labels and Portals Pages have some kind "Back to index button".