Support Google login

This doesn't support setting accounts up for Google login: that still
has to be manually done via the database by setting the email field to
`username@gmail.com@`, where the second `@` denotes that it's using
Gmail login.

If the email field does end in `@`, `getassertion` will note this by
sending `;;@gmail`, to convey that the server is expecting a Google
login token rather than a password.

Upon receiving `;;@gmail`, the client will replace the password box will
with a Google login button, and then send the resulting Google login
token to the server in the `password` field. The server will validate
the "password" using the Google server libraries, and otherwise handle
the login as normal.

Note that Google login requires various features that a paranoid person
might disable; most notably 3rd-party cookies.

Fixes Zarel/Pokemon-Showdown#3394

lib/ntbb-session.lib.php

@@ -179,7 +179,7 @@ class NTBBSession {
 	}
 
 	private function passwordVerifyInner($userid, $pass, $user) {
-		global $psdb;
+		global $psdb, $psconfig;
 
 		// throttle
 		$ip = $this->getIp();
@@ -202,6 +202,18 @@ class NTBBSession {
 			}
 		}
 
+		if (substr(@$user['email'], -1) === '@') {
+			require_once dirname(__FILE__).'/../vendor/autoload.php';
+			$client = new Google_Client(['client_id' => $psconfig['gapi_clientid']]);
+			$payload = $client->verifyIdToken($pass);
+			if (!$payload) return false;
+			if (strpos($payload['aud'], $psconfig['gapi_clientid']) === false) return false;
+			if ($payload['email'] === substr($user['email'], 0, -1)) {
+				return true;
+			}
+			return false;
+		}
+
 		$rehash = false;
 		if ($user['passwordhash']) {
 			// new password hashes
@@ -489,6 +501,7 @@ class NTBBSession {
 				if ($row['password'] && $row['nonce']) {
 					return ';;Your username is no longer available.';
 				}
+				if (substr($row['email'], -1) === '@') return ';;@gmail';
 				return ';';
 			} else {
 				// Unregistered username.