Term merging does not include synonym data

1. Enable the following modules: taxonomy, synonym, term_merge, taxonomy_manager
2. Create a vocabulary with the "Default synonyms field" option checked.
3. Create some terms in the vocabulary.
4. Note the options available when merging terms using the taxonomy module: /admin/structure/taxonomy/{vocabulary_machine_name}/merge
These options need to be available when merging terms using the taxonomy manager module.