[GT-187] Add feature selecting multiple sites #546

Job	Run time
PHPUnit with SQLite, PHP: 5.6	5m 27s
PHPUnit with MariaDB: 10.3, PHP: 5.4, extension: pdo_mysql	5m 42s
PHPUnit with MariaDB: 10.3, PHP: 5.5, extension: pdo_mysql	4m 39s
PHPUnit with MariaDB: 10.3, PHP: 5.6, extension: pdo_mysql	7m 7s
PHPUnit with MariaDB: 10.3, PHP: 7.0, extension: pdo_mysql	15m 9s
PHPUnit with MariaDB: 10.3, PHP: 7.1, extension: pdo_mysql	2m 11s
PHPUnit with MariaDB: 10.3, PHP: 7.2, extension: pdo_mysql	1m 12s
PHPUnit with MariaDB: 10.3, PHP: 7.3, extension: pdo_mysql	1m 12s
PHPUnit with MariaDB: 10.3, PHP: 7.4, extension: pdo_mysql	42s
PHPUnit with MariaDB: 10.3, PHP: 8.0, extension: pdo_mysql	32s
	43m 53s

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