Update AToMiC demo page to support dense search

[dlrudwo1269]

Update AToMiC demo page to support the dense searchers.

One issue is that it is difficult to navigate the image ids when searching for pre-encoded queries for images. I was thinking of adding a index->topic file mapping in the server, and in search_options() we search the topic file to get the image_urls of the matching options to pass to the client side. Did you have any other suggestions on how to better handle this @jasper-xian ?

Demo video: https://youtu.be/ukb4Bc0XzW4