Activity feed: View version snapshots alongside versions

[chillu]

Overview

We are creating a datamodel to capture how changes affect objects in an ownership graph through the https://github.com/open-sausages/silverstripe-versioned-snapshots experimental module.

Design reference: https://projects.invisionapp.com/dsm/silver-stripe/silver-stripe/folder/components/5c68f96ffb26bb0018525d20

Acceptance Criteria

• For objects supported by the existing version history viewer, I can view snapshots which involved this object
• I can view snapshots alongside their versioned entries, through a combined sort
• Both types (versions and snapshots) are sorted as if they were one list
• I can paginate both types (versions and snapshots) as if they were one list
• The viewer continues to support 100k+ version entries
• If the snapshot was created as part of an explicit version for the object, it doesn't show as a duplicate
• Version snapshots show the version number of the object they're associated with
• I am prevented from selecting snapshots in comparisons, because it doesn't make sense until we show an activity feed alongside comparison view
• I can view the details of a snapshot by viewing the version of the object which the snapshot was based on.
• It's clear which point in the feed is currently published for the page record (and what later draft changes are)
• The snapshot detail has a list of changes contained in this snapshot (see activity feed)
• I can revert to a snapshot (based on the date the snapshot was taken)
• For pages: I can preview the object (base on the date the snapshot was taken)

Notes

• We'll need to limit the number of snapshot items to show (particularly when comparing versions with may edits between them, and large ownership structures). We're not implementing pagination of activity feed just yet
• Related: Summary on published snapshots