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/core/settings.h

@@ -133,6 +133,7 @@ struct Values {
     // WebService
     bool enable_telemetry;
     std::string telemetry_endpoint_url;
+    std::string verify_endpoint_url;
     std::string citra_username;
     std::string citra_token;
 } extern values;