[#534] Refactoring / tweaks per MR review.

lib/pleroma/instances/instance.ex

lib/pleroma/plugs/set_requester_reachable_plug.ex

lib/pleroma/web/activity_pub/activity_pub.ex

lib/pleroma/web/activity_pub/activity_pub_controller.ex

lib/pleroma/web/controller_helper.ex

lib/pleroma/web/ostatus/ostatus_controller.ex

lib/pleroma/web/salmon/salmon.ex

lib/pleroma/web/websub/websub.ex

lib/pleroma/web/websub/websub_controller.ex

priv/repo/migrations/20190123125546_create_instances.exs