Users: When creating the first user on installation, populate the Website...

Users: When creating the first user on installation, populate the Website profile field with the site URL.

Skip setting the field if the user already exists, which is the case when the user tables are being shared among multiple sites.

Props EFAREM, eclare, darrenlambert, zachflauaus, viralsampat.
Fixes #35778.
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git-svn-id: https://core.svn.wordpress.org/trunk@46789 1a063a9b-81f0-0310-95a4-ce76da25c4cd