Display the most "recent" assignment

Rather than displaying the first assignment on load, the most "recent" assignment is now displayed.

The logic is as follows: we start showing (or "featuring") the assignment 3 days before it's due and then keep showing the stats/info for the assignment until 3 days before the next assignment is due. That way, instructors can see when students start handing in the assignment and the progress as TAs are marking them.

Tests
* manually tested by creating new assignments
* ran all rspec tests (all passed)