Added migration to make the combination of slug+domain unique

module/Core/config/entities-mappings/Shlinkio.Shlink.Core.Entity.ShortUrl.php

@@ -28,7 +28,6 @@ $builder->createField('longUrl', Type::STRING)
 
 $builder->createField('shortCode', Type::STRING)
         ->columnName('short_code')
-        ->unique()
         ->length(255)
         ->build();
 
@@ -66,3 +65,5 @@ $builder->createManyToOne('domain', Entity\Domain::class)
         ->addJoinColumn('domain_id', 'id', true, false, 'RESTRICT')
         ->cascadePersist()
         ->build();
+
+$builder->addUniqueConstraint(['short_code', 'domain_id'], 'unique_short_code_plus_domain');