[GT-187] Add feature selecting multiple sites #555

Job	Run time
PHPUnit with SQLite, PHP: 5.6	6m 24s
PHPUnit with MariaDB: 10.3, PHP: 5.4, extension: pdo_mysql	9m 0s
PHPUnit with MariaDB: 10.3, PHP: 5.5, extension: pdo_mysql	10m 1s
PHPUnit with MariaDB: 10.3, PHP: 5.6, extension: pdo_mysql	5m 33s
PHPUnit with MariaDB: 10.3, PHP: 7.0, extension: pdo_mysql	16m 5s
PHPUnit with MariaDB: 10.3, PHP: 7.1, extension: pdo_mysql	2m 14s
PHPUnit with MariaDB: 10.3, PHP: 7.2, extension: pdo_mysql	1m 18s
PHPUnit with MariaDB: 10.3, PHP: 7.3, extension: pdo_mysql	1m 17s
PHPUnit with MariaDB: 10.3, PHP: 7.4, extension: pdo_mysql	50s
PHPUnit with MariaDB: 10.3, PHP: 8.0, extension: pdo_mysql	42s
	53m 24s

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