Implement video -> GIF conversion.

Not ready to be merged yet, but the conversion is working.

Stuff still to be done:
- [x] UI for triggering the conversion (currently just clicking on the video does it)
- [x] UI for saving/uploading to imgur/whatever? (Chrome would allow for a "click video -> file is saved" type workflow, I think this is harder to accomplish in Firefox)
- [x] Testing to ensure this works properly across different browsers (only tested in Chrome thus far)