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

[dlrudwo1269]

@jasper-xian @justram would appreciate a review here!

[jasper-xian]

Not sure I'm the right person to comment on this, but the way we did pre-encoded image queries was something like this:

Topics: {'id': , 'title': }
Pre-encoded queries: {'id': , 'text': , 'embedding': }

So you could use the image-id to search for the right image. I'm not sure what this mapping stuff relates to in general though, but I think the app looks good! @justram can probably comment more about it