Update REST Grammar around MediaNext/MediaPrev/MediaCount related to annotator visuals

[bctcvai]

The annotator has a nice navigation feature to go to previous or next media and see image previews for the elements.

Unfortunately, the REST grammar is suboptimal to make these visualizations. There is a seperate call to MediaPrev, MediaNext as well as MediaCount; then an accessor to Media detail view for each ID to fetch presigned URLs for the hover display.

MediaPrev/MediaNext should be deprecated and a new endpoint (MediaNavigation) should return back:

• The count you'd get from MediaCount
• The previous media record (optionally with presigned URLs)
• The next media record (optionally with presigned URLs)

This should cut down on excessive REST calls when loading the annotator view.