Move synonym API to EXT:solr Solr service class

Keep third party library untouched.
Forward GET parameters to their modules
Reset arguments when forwarding to a module's indexAction()
Fix phpDoc for type hints
Add parameter to retrieve synonyms for a specific base word
Move schema API changes to EXT:solr SolrService class
Add support for HTTP DELETE requests
Add button to remove synonym mapping
register valid actions for synonyms module