Optimize Upvoted Users.

[ngwattcos]

Currently, upvotedUsers in FireDiscussionQuestion is a string array. This translates to an array in Firestore, which is essentially a map with the index of each upvoted user as the key. I'm not 100% on this but I believe this translates to an O(n) lookup time.

If we convert upvotedUsers: Record<string, {}> to mimic a set (and have Firestore updated to store upvotedUsers as a map) then we may see some performance benefit :')