AO3-6268 Fix All challenge owners/moderators get CCed in "Assignments sent" notification #4511

Job	Run time
rake db:otwseed	1m 18s
rspec spec/controllers	11m 43s
rspec spec/models	5m 50s
rspec --exclude-pattern 'spec/{controllers,models}/*/.rb'	7m 25s
cucumber features/admins	5m 58s
cucumber features/bookmarks	4m 37s
cucumber features/collections	5m 41s
cucumber features/comments_and_kudos	6m 26s
cucumber features/gift_exchanges	6m 23s
cucumber features/importing	2m 50s
cucumber features/other_a	9m 34s
cucumber features/other_b	6m 37s
cucumber features/prompt_memes_a	4m 5s
cucumber features/prompt_memes_b	3m 28s
cucumber features/prompt_memes_c	5m 1s
cucumber features/search	6m 42s
cucumber features/tag_sets	3m 44s
cucumber features/tags_and_wrangling	5m 52s
cucumber features/users	4m 58s
cucumber features/works	13m 56s
	2h 2m 8s

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