Fix adding tag to entries from other people

I've also limited tag length to 20 chars (and limit adding more than 5 tags at once)

src/Wallabag/CoreBundle/Form/Type/NewTagType.php

@@ -10,6 +10,8 @@ use Symfony\Component\OptionsResolver\OptionsResolver;
 
 class NewTagType extends AbstractType
 {
+    public const MAX_LENGTH = 40;
+
     public function buildForm(FormBuilderInterface $builder, array $options)
     {
         $builder
@@ -17,6 +19,7 @@ class NewTagType extends AbstractType
                 'required' => true,
                 'attr' => [
                     'placeholder' => 'tag.new.placeholder',
+                    'max_length' => self::MAX_LENGTH,
                 ],
             ])
             ->add('add', SubmitType::class, [