in order to fill _milestone_id columns

The name is reserved for more common association methods.

Tracking which milestone the milestone dates are sourced from

Rationale: match closer to web UI where users without epic edit access can not see those fields.

Expose new date fields.
Translate start_date and end_date params as '_fixed' for backward compatibility.

1. When milestone's dates changes
2. When issue changes milestone
3. When epic's issues change
4. When Epic changes date fields

Convert use of any_instance as it does not work with prepend

Those are composite fields managed by the system so no direct updates should be allowed