Fix build on platforms without std::string_view

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.

src/web_service/verify_login.cpp

@@ -2,7 +2,7 @@
 // Licensed under GPLv2 or any later version
 // Refer to the license.txt file included.
 
-#include <json.hpp>
+#include "web_service/json.h"
 #include "web_service/verify_login.h"
 #include "web_service/web_backend.h"