* WIP Implement polymorphic sitetree link tracking
* Update unit tests
Merge SiteTreeTrackedPage into SiteTree directly
* Fix bugs and issues
* Fix support for file link tracking
* Add missing use
* Add back deprecated extension
* Remove obsolete belongs_many_many
* Update deprecations
* BUG Ensure non-SiteTree records support link tracking
* Safer changed check
* Shift file tracking test to assets module
* Better check for live stage on versioning
* Deprecate method
* Cleanup virtualpage
* Clear records on delete
* Ensure upgrade task occurs on draft
* fix linting
API Decouple File and ErrorPage
API Link tracking is now only performed on stage (in lieu of versioned relationships)
API Refactor versioned API methods out of SiteTree and into Versioned
Before it would be applied on the fly during the rendering of the
HtmlEditorField, and only be written to the database during the
subsequent write.
We just shift the behaviour to apply just-in-time.
This code has a dependency on SiteTree, so it fits much better in the
cms module.
Abstracted away the content parser so the same code can be reused both
in the render phase (to highlight the links) and in the write phase
(storing information about broken links and references).
