SPIKE: Exploring the WP state change machine to create custom transitions

Breaking up the technical research required for our Workflows solution into following constituent pieces:

In this issue, we'd like to explore what does the WP state change machine entail for creation of custom transitions.

## Requirements
1. The ability to create custom transitions between states created in #46
2. The ability to properly design/specify what each transition does eg: Only the author can take a published post to a 'Back to Draft' state to make changes or a 'Under review' state should not be allowed to publish
3. The ability to specify user permissions for certain transitions eg: Only the author or their superior or admin has the permissions to 'Archive' a post
4. The ability to block certain transitions eg: An archived post cannot be taken to a draft state directly. It needs to be unarchived first.

**Timebox: 1 day**