Add snippeting and fix some UI bugs.

[nsthorat]

• Snippets now are chosen by determining a subset of spans that cross the .5 threshold. Then we sort the rest, and add them until we reach the budget (~500 characters).
• Fix the bug where scrolling the item container keeps the hover spans around.
• Make the right hand side have max height = the size of the media container. Now it scrolls. If it is less than the height, it is sticky.
• Remove metadata that is a child of a span. It gets rendered with the media.

snippets.mp4