Create an OnThisDayDataController (this will be shared across multiple features), add a fetchData method that makes the on this day API call for a given WMFProject and date (for example: https://en.wikipedia.org/api/rest_v1/feed/onthisday/events/2/26). Note that not all WMFProjects will support on this day, this method needs to throw an error if it doesn't support it. Return response struct. Mock this call and add unit tests. (WMFGrowthTasksDataControllerTests is a good simple example for how we do this).