Student can review past coaching sessions

[ti2ger92]

Add the image (final state of the whiteboard) to the session object in the MongoDB.
Allow the student to retrieve the saved images in their session history.
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See wireframes - students can see a list of past sessions (query DB to see all sessions with that student as the student id) , after selecting a session from the list, can see the chat log )
eventually - will also have final state of whitebaord + any images which were saved during the session (not necessary for pilot launch)