[GT-187] Add feature selecting multiple sites #551

Job	Run time
PHPUnit with SQLite, PHP: 5.6	5m 18s
PHPUnit with MariaDB: 10.3, PHP: 5.4, extension: pdo_mysql	4m 50s
PHPUnit with MariaDB: 10.3, PHP: 5.5, extension: pdo_mysql	6m 55s
PHPUnit with MariaDB: 10.3, PHP: 5.6, extension: pdo_mysql	5m 45s
PHPUnit with MariaDB: 10.3, PHP: 7.0, extension: pdo_mysql	17m 14s
PHPUnit with MariaDB: 10.3, PHP: 7.1, extension: pdo_mysql	1m 53s
PHPUnit with MariaDB: 10.3, PHP: 7.2, extension: pdo_mysql	1m 5s
PHPUnit with MariaDB: 10.3, PHP: 7.3, extension: pdo_mysql	1m 6s
PHPUnit with MariaDB: 10.3, PHP: 7.4, extension: pdo_mysql	39s
PHPUnit with MariaDB: 10.3, PHP: 8.0, extension: pdo_mysql	34s
	45m 19s

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