Bulk Editing Previously Published Posts from Draft to Published Resends Notification Email

Steps to reproduce:
1. Publish a post. It sends a notification email as it should.
2. Change that post to a draft.
3. Switch that post back to "Published" using the Bulk Edit Tool (I think this happens with Quick Edit as well). Provided the Subscription Module is activated, a new email is sent out.

We should probably only send one post notification out when the post is originally published and not if the status is flipped back and forth. Confirmed changing status on WP.com does not generate a new email.

Start by tracing the Bulk Edit Tool and Quick Edit status changes into the Subscription Module's post notification path. Reproduce the sequence of publishing, drafting, and republishing a post, then verify that only the original publication sends an email. The issue names no files or tests, so locate the relevant entry points before defining coverage.

Written by the indexing model from the issue text.