Add API endpoint to check if user login token is correct

While /createuser could be used to check if the user exists, there is no way to know if the login token provided by the user is correct. Hence, I propose an endpoint named /checkuser that accepts the username and logintoken, which then returns {success: true} if they match up; if not, provide a reason/failure code that could be used by clients to determine which error message to show in the login dialog.