sets full-page alert banner to center align, adds ::backdrop #514

Job	Run time
tests / Install LocalGov Drupal (3.x, ~10.0, 8.1)	1m 37s
tests / Install LocalGov Drupal (3.x, ~10.0, 8.1)	1m 28s
tests / Install LocalGov Drupal (3.x, ~10.0, 8.1)	55s
tests / Install LocalGov Drupal (3.x, ~10.0, 8.2)	1m 25s
tests / Install LocalGov Drupal (3.x, ~10.0, 8.2)	1m 3s
tests / Install LocalGov Drupal (3.x, ~10.0, 8.2)	44s
tests / Install LocalGov Drupal (3.x, ~10.0, 8.3)	1m 11s
tests / Install LocalGov Drupal (3.x, ~10.0, 8.3)	1m 3s
tests / Install LocalGov Drupal (3.x, ~10.0, 8.3)	38s
tests / Static analysis checks (3.x, ~10.0, 8.3)	20s
tests / Static analysis checks (3.x, ~10.0, 8.3)	18s
tests / Coding standards checks (3.x, ~10.0, 8.3)	19s
tests / Coding standards checks (3.x, ~10.0, 8.3)	12s
tests / PHPUnit tests (3.x, ~10.0, 8.1)	6s
tests / PHPUnit tests (3.x, ~10.0, 8.1)	2m 0s
tests / PHPUnit tests (3.x, ~10.0, 8.2)	10s
tests / PHPUnit tests (3.x, ~10.0, 8.2)	2m 1s
tests / PHPUnit tests (3.x, ~10.0, 8.3)	7s
tests / PHPUnit tests (3.x, ~10.0, 8.3)	1m 51s
tests / PHPUnit tests	0s
tests / Static analysis checks	0s
tests / Coding standards checks	0s
	17m 28s

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