Taking them by const reference isn't advisable here, because it means
the std::move calls were doing nothing and we were always copying the
std::string instances.
json.hpp wants it, but it isn't always available. Rather than patch
json.hpp directly to remove the dependency, provide a json.h wrapper
header that subs in std::experimental::string_view using preprocessor
magic. All the consumers of json.hpp are in src/web_service, so the
wrapper header is placed there as well.
