[Speedgrader] Add date when students view feedback for an assignment to the Canvas API

[woodsdm2]

In the Canvas SpeedGrader you can see whether or not a student has viewed the feedback added to files submitted with an assignment.  The Canvas API provides data about whether or not feedback has been viewed, but NOT the details of the date when it was viewed.  This information should be added to the API.

There is a lot of research showing that it is important for students to engage with feedback to continue the learning cycle so that feedback feeds forward as improvements on future assignments.

The step in trying to improve the feedback I offer and students' engagement with it is seeing whether students view the feedback, and when they view it.  Viewing this info in the SpeedGrader is a slow process, especially for larger classes, so the data needs to be accessed using the API.  While having a yes/no indication of whether a student has viewed the feedback has some value, having the actual date is important so that I can track the time between when feedback was released and when it was viewed and look for correlations to student grades, etc.  Also, seeing the specific dates that students viewed feedback will let me see the effectiveness of different efforts I make to promote feedback viewing - similar to being able to view the response rate to a messaging campaign.

Since the date when the feedback was viewed is already being displayed in the SpeedGrader, I would hope that it would be easy to make this info available through the API.