Object detection only run on ~22 frames of video

[jharwig]

Videos are processed into about 22 frames for video scrubbing, and then those frames are run through OpenCV for detecting objects. But for longer videos we should run object detection on more frames, even if we don't keep them around afterwards.

[sfeng88]

We were calculating the number of frames to extract by dividing the duration by 20. Now we are extracting a frame per every second and OpenCV runs on the extracted frames.

Commit: fd88f7c