WebService: Verify username and token (#2930)

* WebService: Verify username and token; Log errors in PostJson

* Fixup: added docstrings to the functions

* Webservice: Added Icons to the verification, imrpved error detection in cpr, fixup nits

* fixup: fmt warning

src/web_service/verify_login.cpp

@@ -0,0 +1,28 @@
+// Copyright 2017 Citra Emulator Project
+// Licensed under GPLv2 or any later version
+// Refer to the license.txt file included.
+
+#include <json.hpp>
+#include "web_service/verify_login.h"
+#include "web_service/web_backend.h"
+
+namespace WebService {
+
+std::future<bool> VerifyLogin(std::string& username, std::string& token,
+                              const std::string& endpoint_url, std::function<void()> func) {
+    auto get_func = [func, username](const std::string& reply) -> bool {
+        func();
+        if (reply.empty())
+            return false;
+        nlohmann::json json = nlohmann::json::parse(reply);
+        std::string result;
+        try {
+            result = json["username"];
+        } catch (const nlohmann::detail::out_of_range&) {
+        }
+        return result == username;
+    };
+    return GetJson<bool>(get_func, endpoint_url, false, username, token);
+}
+
+} // namespace WebService