Add foreign key constraint from table 'idea_student' to table 'program' (#153)

Fixes #66

Reviewed-on: #153
Reviewed-by: Andreas Svanberg <andreass@dsv.su.se>
Co-authored-by: Tom Zhao <tom.zhao@dsv.su.se>
Co-committed-by: Tom Zhao <tom.zhao@dsv.su.se>

core/src/main/resources/db/migration/V7__add_fk_to_program_from_student_idea.sql

@@ -0,0 +1,4 @@
+alter table idea_student
+    add constraint fk_idea_student_program_id
+        foreign key (program_id) references program (id)
+            on update cascade on delete set null;