Disable mod source change on edit (#301)

This will:
- Disable mod source select list on mod edit as it doesn't make sense to
change Steam Workshop mod to Directory mod and vice versa and it was
never fully supported when mod was assigned to groups or lists.

src/Form/Mod/ModFormType.php

@@ -27,9 +27,14 @@ public function __construct(
 
     public function buildForm(FormBuilderInterface $builder, array $options): void
     {
+        /** @var ModFormDto $modFormDto */
+        $modFormDto = $options['data'];
+        $modExists = null !== $modFormDto->getId();
+
         $builder
             ->add('source', ChoiceType::class, [
                 'label' => 'Mod source',
+                'disabled' => $modExists,
                 'choices' => [
                     'Steam Workshop' => ModSourceEnum::STEAM_WORKSHOP->value,
                     'Directory' => ModSourceEnum::DIRECTORY->value,