Hide revision dependency stories from feed/notifications

Summary:
See PHI1134. Generally, "alice added a dependent revision: ..." isn't a very interesting story. This relationship itself is valuable, but the creation of the relationship is usually pretty obvious from context.

In the specific case of PHI1134, various scripts are racing one another, but I don't think this story is of much value in the general case anyway.

Test Plan: Edited parent/child revisions, no more feed stories.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D20437