Add Video Annotator feature

[meta-paul]

Add a convenient Task Generator for video annotation.

Key features:

• User can mark multiple video segments along multiple tracks, and annotate each one
• For each segment, researcher can specify (via JSON config) desired annotation input as a list of form fields
• VideoAnnotator JSON config works similar to FormComposer (including pre-signed S3 URLs, and support for custom JS triggers)
• Workers inputs are downloadable in JSON, and when viewing a unit are also available in WebVTT format

Other related improvements:

• Now in TaskReview app there's ability to view completed units without assessing them