Simplified user info update APIs, DB and frontend to remove separate update functions for each property. Automatically choose preferred username for users as they register, based on display name.

frontend/auth/admin/AdminApp.vue

@@ -808,7 +808,7 @@ async function submitDialog() {
 
       // Close dialog immediately, then perform async operation
       closeDialog()
-      apiJson(`/auth/api/admin/users/${user.uuid}/display-name`, { method: 'PATCH', body: { display_name: name } })
+      apiJson(`/auth/api/admin/users/${user.uuid}/info`, { method: 'PATCH', body: { display_name: name } })
         .then(() => {
           authStore.showMessage(`User renamed to "${name}".`, 'success', 2500)
           onUserNameSaved()