Allow editing usernames

[rmccue]

WordPress doesn't allow editing usernames. This does not appear to have any real technical requirement, as WP uses IDs to track almost every piece of data.

The only things this affects (so far as I know) is URLs and the super-admin setting. The former we can warn users about, and we can block super-admins from having their usernames updated if we have to, or just update the setting automatically for them.

WP-CLI may also block this in its own code, so would need to ensure that this applies both across the UI and the CLI.