When non-administrators click "Change Username", explain why they can't continue

Summary: Ref T13420. This workflow currently dead-ends for non-administrators. Instead, provide explanatory text.

Test Plan:

• Clicked "Change Username" as an administrator, same workflow as always.
• Clicked "Change Username" as a non-administrator, got explanatory text.

Maniphest Tasks: T13420

Differential Revision: https://secure.phabricator.com/D20827