Add unique (#685)

Usernames have to be unique, trying to make a new user with an existing username results in a 500, this fixes it.

app/Filament/Resources/UserResource/Pages/ListUsers.php

@@ -91,6 +91,7 @@ protected function getHeaderActions(): array
                             TextInput::make('username')
                                 ->alphaNum()
                                 ->required()
+                                ->unique()
                                 ->maxLength(255),
                             TextInput::make('email')
                                 ->email()