Introduce link_external_site as course attribute (#1555) (#1629) #9649

Job	Run time
wagon_tests / seeds	3m 24s
wagon_tests / rubocop	1m 49s
wagon_tests / specs	7m 16s
wagon_tests / feature specs	9m 20s
wagon_tests / notify_statuscope / notify-statuscope	4s
	21m 53s

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