Add captions flow into File Review page.

[mbusch3]

Caption Editing widget

File Review now includes CAPTION EDITING for media files (audio and video)!

Features

• Existing caption information is loaded from and saved back to Canvas.
• Each file can have multiple tracks associated with it (like "English Captions", "English Descriptive Audio", and "Spanish Captions"). Checks are in place to make sure that no two tracks have the same Language + Type combination.
• Cues can be added at the current timestamp or directly before/after existing captions.
• Live preview of the video includes the current captions/subtitles.
• The waveform visualizer shows current text and accurate timestamps.
• Caption timing can be edited by drag-and-dropping inside the waveform visualizer.
• Drag-and-drop timing also includes "snapping" to align exactly with nearby captions when within 0.25 seconds.
• Media files have an accurate progress bar when downloading.
• Media files are cached so long as the user stays on the File Review page. Closing the editing widget or going to the "next file" doesn't immediately dump the file in case the user wants to go back.
• All controls are keyboard-accessible.

Testing

This PR can only be tested on the TEST server, since it requires media files be uploaded and processed, and that's where we have that. These media files should show up in the Review Files page. Clicking any audio or video type file will open the new captioning widget instead of the Review-and-Replace widget.