Replace usage of UrlEntityDto by String

[php-coder]

UrlEntityDto holds id and slug members that are used for building URLs. After migration to URLs with slugs (#205, #206, #207) we don't use id and I think that we can replace UrlEntityDto by String.

The following methods should be updated:

• CategoryService.add()
• CountryService.add()
• CollectionService.addToCollection()
  • CollectionDao.addSeriesToCollection(): replace argument collectionId by collectionSlug
• CollectionService.removeFromCollection()
  • CollectionDao.removeSeriesFromCollection(): replace argument collectionId by collectionSlug