Detect package change and send out email

Summary:
For package creation and deletion, send email to all the owners For
package modification, detect important fields such as owners and paths, and then
send out emails to all owners (including deleted owners and current owners)

Also start using transaction for package creation/deletion/modification.

Test Plan:

• tested mail creation and deletion
• tested modification to auditing enabled, primary owners, owners, paths

Reviewers: epriestley, nh, vrana

Reviewed By: epriestley

CC: prithvi, aran, Koolvin

Differential Revision: https://secure.phabricator.com/D2470