automatic vanishing of older projects

two years after a student cohort has left the school, their projects are automatically set to "blocked".

for example: mmt-b2024: cohort graduates in sept 2026, mmp's will be blocked after sept 2028 unless a person attends the master, then all their projects will be visible for the time of their masters + two years. for example: one of mmt-b2024 starts the master in mmtb-m2028, graduates in september 2030. all of their projects will be blocked after september 2032