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, "Staged" is probably better) 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"

[emteknetnz]

I've spent a decent chunk of time looking at this, probably a goods days worth. IMO this is in the "too hard" basket, I'm not convinced the effort to benefit ratio is worthwhile here, and there are other things that we could spend our time on that would yield more for the time invested

The Versioned class is really a large tangled knot of string and splitting it all apart into a pair of extensions, which, while it should make for cleaner code, it's a lot of work. I also can't help thinking that the split won't actually be that clean anyway as you have things like the "WasPublished" column on the _Versions table, which seems like it will naturally result in lots of if ($this->owner::has_extension(Publishable::class)) ... within the Versioned extension, somewhat negating the point of splitting them, and possibly increasing complexity rather than reducing it.

I made a crude attempt to split everything into Versioned / Staged / Mode (temporary thing with both Versioned + Staged code), and split the giant Versioned class into a bunch of traits so that I could getting a better view of what's going on, before eventually giving up which link to this issue. The silverstripe/versioned PR is the relevant one in case anyones intereseted