Make sure deferred students are represented in the class in which they attend not apply

[jamiecmontgomery]

If a student defers, they are kept in the data for the year they applied, not the year they enroll. I fixed this for 2022 (MESM class of 2024) but did not retroactively update this.

Additionally, we sometimes have students who defer and then never enroll. We can adjust for this in the data cleaning by joining the application dataset to the previous class lists.