Change the BackLinks relation in SiteTree to a polymorphic many_many relation

[sabina-talipova]

Description

In SiteTree, the BackLinks relationship should be turned into a polymorphic many_many relationship (using a many_many through relation as documented).

SiteTree::BackLinksTracking() would then be redundant - you'd just get the backlinks directly from the polymorphic relation.

See:

silverstripe-cms/code/Model/SiteTree.php

Lines 1961 to 1962 in 58a2d58

	// @todo - Implement PolymorphicManyManyList to replace this
	$list = ArrayList::create();

Related

• File::BackLinkTracking() - Implement PolymorphicManyManyList to replace ArrayList silverstripe-assets#575