Separate "stages" logic and concept from "versions" logic and concept

[GuySartorelli]

Right now most of the logic for both stages (draft, published) and versioning is held inside the same extension - Versioned.

With that extension, you can have the following:

1. Records which record their version history, but do not have a separation between "Draft" and "Published"
2. Records which both record their version history, and also have a separation between "Draft" and "Published"

It isn't possible to have a separation between "Draft" and "Published" without recording version history.

What's more, the way these two separate concepts have been weaved together results in a lot of complexity both in the code itself and in the way developers need to reason about the code.

Related issues

• Versioned.versioned vs Versioned.versionedstaged is confusing #79
• Refactor Versioned to use more composition #421

Acceptance criteria

• Versioned and Publishable (think about naming) have their own separate extensions
• It's decided where the Publishable extension should live, e.g. in silverstripe/framework or silverstripe/versioned
• All core classes with Versioned extension have the Publishable extension added too
• Consideration is given to alleviate upgrade pain for projects since the Publishable extension must be added in various places e.g. has_extension(Versioned::class) calls
• The Versioned extension no longer has "modes" it is either applied or it is not.
• Any DataObject can have one, both, or neither of the Publishable and Versioned extensions, i.e. the two extensions combine nicely (i.e. when you publish you create a new version), and can be used separately

Notes

• Current modes are "use stages and version history", and, "only use version history". There is currently a lack of an "only use stages " mode"