Publish create object stories into Asana sort of, but not really

Summary: Ref T2852. Current code works fine, but although we want to drop creation stories, we really only want to drop the story text, not the other effects of the creation story. Also generalize this mechanism so we don't have Asana-specific code in the publishers.

Test Plan: Used `bin/feed republish` to publish creation and non-creation stories. Verified creation story published no text.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2852

Differential Revision: https://secure.phabricator.com/D6639