Fix Synonym CRUD

Adjusting attributes since API has changed
Move SearchReferences base controller under Synonyms namespace

Because it is not being used outside it, so makes sense for it to live
there
Log GOVUK-Request-Id header
Merge pull request #13 from alphagov/58810758-synonyms-refactoring

58810758 synonyms refactoring
Merge pull request #16 from alphagov/log_govuk_request_id

Log GOVUK-Request-Id header