Handle duplicate activities - added as a presenter activity (and other activities?)

[renintw]

Ref: WordPress/wordpress.org#84

Description

In the comment,
Both a) Updating a post and b) Switching back to draft and publishing a post again would trigger notify_workshop_presenter and cause duplicate activities.

Note: Probably can be handled in Profile API.

Acceptance Criteria

1. When updating a post, do not add duplicate activity if there's already one on a user's profile.
2. When switching a post back to draft and publishing it again, do not add duplicate activity if there's already one on a user's profile.

Further Discussion

1. Do we also want to handle other duplicate activities? For example, switching a post back to draft and publishing again, the activity Wrote a new workshop, ...... will be added to the user's profile again, causing duplicate.