Provide a mechanism for selecting which source is previewed

[thatbudakguy]

There are a couple of directions we could take this.

In GeoBlacklight, there is a hardcoded method that lists the formats to use in order of preference. That is, if both a COG source and a IIIF source are present in dct_references_s, the COG source will take precedence and that's the only one that the viewer will render.

We could start with just taking the same approach and hardcoding things for now. But...there are other options.

1. Allow the user to configure the order of preference. This is sort of the case in GeoBlacklight now, because you can subclass ItemViewer and override #viewer_preference in order to change your own installation's preference order.
2. Have all of the sources available in the viewer, and let the user toggle between different sources to preview if they want. That is, in the previous example, show the COG initially but have some sort of toggle (tabs? buttons?) to view the IIIF version instead.

There's more to think about here re: how this could work.

[thatbudakguy]

#44 got us to parity with GeoBlacklight (i.e. a hardcoded preference list of sources, where the first match is used).