Fixed bug in display_name validations

- [ ] Big fix for issue rapidftr/tracker#137, deleted validate_unique_display_name method from Field class because it makes senses to have fields with duplicate display names.
- [ ] Minor refactor of validate_unique_name method, deleted commented out code.
- [ ] Introduced unit tests for child search#fulltext_by method, as we have configured solr to perform multilingual searches.