As an auditor, I want an option for custom fields to be immutable and once the value is set, it cannot be changed so that I can add governance management data to my user stories.

- Custom fields can be optionally immutable
- Immutable fields can be optionally required upon work item creation
- Once set, the value of an immutable custom field cannot be changed (similar to log entries)
- BONUS: standard fields can be made immutable, except where doing so negatively impacts the application, such as Status

For examples:
- original requester of a user story is preserved
- an immutable fields of Date/Time field preserves the date/time of when the value was set, creating a checkpoint time