Follow up: Add foreign key constraints to epics table for date sourcing references

Summary

This is a follow up issue for Epics table contains 2 fields referencing milestones, but missing foreign key constraints. We should add the FKs for start_date_sourcing_milestone_id and due_date_sourcing_milestone_id.

Data Validation

If we anticipate existing data that would violate FKs, we will have to update these records (set null perhaps) before creating the FK constraint.