[Spec] Location drafts

Discussion on spec for location drafts in SPI.

API: This is just a first step, basically introducing support for Location drafts Content on version 1, to fix issue with preview.

SPI: Further steps will be needed now/later like exposing a method on LocationHandler to change status, which will be more optimal then setting it on Location update struct. PS: PR removes $rootLocationId as it can easily be done in API as shown here.
> So, idea was that we can stage Location drafts in the future, but for now it will indeed only be used for first version.

How would this work exactly, adding draft Locations only to draft Content, or? With this in mind we might want to keep `$status` on UpdateStruct, or introduce something like `LocationService::publishLocation()`.

> Q1: How does `eznode_assignment` work for version after first published version?

It does store Content's version, but it new stack we don't really use it, we just hide it in the Gateway and update it only for Legacy Stack benefit.

> Q2: Is there any value in staging locations? if not this can argument can be changed to bool for instance.

Still not quite following this -- `$rootLocationId` just limits on the subtree.