Modularize Owners package transactions

Summary: Converts Owners package transactions to modular transactions.

Test Plan:

• created a new package
• edited all simple properties from the web ui
• checked that project and user owners were added as reviewers appropriately to new diffs
• inspected the change details for various types of path add / remove / update / reorder changes

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D16651