Refactor post_updated and wp_insert_post callbacks

[ilicfilip]

Context

This PR solves the issue we have described in #122 .

While testing after refactoring I found a bug, the refactored method was working fine but the problem was in transition_post_status method which allowed multiple update activities to be added.

I believe that was wrong since in the post_updated method we had a check for not adding update activity if it was already added recently. transition_post_status was missing that check so it allowed user to add activities by simply changing the post status, for example from publish to draft and back from draft to publish.

If I was wrong then only the 2nd commit needs to be reverted.

Quality assurance

• I have tested this code to the best of my abilities.
• I have added unit tests to verify the code works as intended.
• I have checked that the base branch is correctly set.

Fixes #122

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