LinkEntityDto: remove id member

[php-coder]

LinkEntityDto holds id, slug and name 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 remove it.

[php-coder]

Also we should find&replace all occurrences of category.id and country.id in Thymeleaf templates.

[php-coder]

Another idea is to replace it by EntityWithSlugDto.