Move visibility into own module.

lib/pleroma/gopher/server.ex

lib/pleroma/web/activity_pub/activity_pub.ex

lib/pleroma/web/activity_pub/activity_pub_controller.ex

lib/pleroma/web/activity_pub/transmogrifier.ex

lib/pleroma/web/activity_pub/visibility.ex

lib/pleroma/web/federator/federator.ex

lib/pleroma/web/mastodon_api/mastodon_api_controller.ex

lib/pleroma/web/ostatus/ostatus_controller.ex

lib/pleroma/web/streamer.ex

lib/pleroma/web/twitter_api/twitter_api_controller.ex

test/web/activity_pub/visibilty_test.exs