Implement Feedback Servlet

[kaylaleung]

Notes:

• Storring additional upvote and downvote information for potential future use?
• Also I believe datastore stores numerical fields as longs, hence the long casts.

[irisliu77]

if we only wants total votes in the frontend, i think we can only keep one totalvotes field

[kaylaleung]

if we only wants total votes in the frontend, i think we can only keep one totalvotes field

Yeah, for this PR, having multiple fields to track upvote and downvote isn't absolutely necessary, but the idea is that the list of upvotes and downvotes will be helpful in the future if we're tracking the list of people who have already upvoted vs downvoted and prevent toggling multiple times