'Change dashboard or filter ownership' conflates user names and user keys

To reproduce:

• Create a user and then change their username.
• Create a dashboard/filter under that user's ownership.
• Go to the 'Change dashboard or filter ownership' built-in script and select the user as the 'From' user.

Expected: The dashboards/filters list(s) will populate.

Actual: Nothing is populated and there is a 500 error from the script's /params endpoint.

The problem arises because the user select dropdown produces the user's key as its value, but the back-end script looks up user information by username. Usually these are the same but when they aren't it causes the error.