Multiple AV file support for video events

[camdendotlol]

Summary

• adds the ability to switch between AV file, when applicable, on video events
  • uses a new Canvas-based component to display the video thumbnails (my Topsters experience came in handy!)
  • the thumbnail will not be shown if anything goes wrong when trying to load it (the title and duration will still be shown)
• fixes an issue where annotations from all files were being shown on initial page load instead of just the ones for the current file

Testing

The different cases listed in #63 also apply here.

Screenshot

(The second AV here begins with a black screen - the thumbnail is correct)