feat: add chart for full and partial video views (FC-0051)

[SoryRawyer]

This adds a chart to show the number of full and partial views of course videos. This builds on the watched video segments query and compares the number of segments viewed by a learner to the expected number of segments in the video. The number of segments is calculated by first taking ten seconds off the video duration and then calculating the number of segments based on that shortened duration. This is to account for learners who, say, watch the entire video but skip the end credits.

Here is a screenshot of the chart:

[openedx-webhooks]

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[Ian2012]

Waiting on @bmtcril opinion

[bmtcril]

Seems like as good a start as we can ask for, we'll run it through the user tests and performance to see how it all shakes out!

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