adding notify_email setting for trigger emails

config/config.exs

config/test.exs

docs/config.md

lib/mix/tasks/pleroma/instance.ex

lib/mix/tasks/pleroma/sample_config.eex

lib/pleroma/emails/admin_email.ex

lib/pleroma/emails/user_email.ex

test/tasks/instance.exs

test/web/admin_api/admin_api_controller_test.exs

test/web/mastodon_api/mastodon_api_controller_test.exs

test/web/twitter_api/twitter_api_controller_test.exs

test/web/twitter_api/twitter_api_test.exs