Add/modify dining endpoint to show differences between daily menus

[ezwang]

When looking at the dining hall menus, users are often familiar with what is served daily, and want to know what items are different for that particular day. For instance, in 1920 commons people want to know what the Expo item is, since it changes every day.

We should use Redis to cache the dining hall menu for the previous day, compare it to the current day's dining menu, and return all of the items that are different. In the mobile app, these should be displayed at the top for user convenience.