DIP-220 Identifier retirement

[wesbiggs]

Problem

Per #220, retiring a user identifier is an important aspect of user control.

Solution

Add a Retire Identifier Operation and corresponding State Change Record. A description of the rationale and implications of identifier retirement is also added to the Identity page.

Change summary:

• [ ] Added discussion to Identity page
• [ ] Updated Operations and State Change Records

Steps to Verify:

1. Generate and review site.