PostBlock/NotificationBlock: Import blocked posts from New XKit

[marcustyphoon]

Concept

By copying code from #1279 and new-xkit/XKit#2135, we should be able to implement New XKit settings migration for these features relatively(?) easily.

May require #1661 in order to semi-revert 6fd9963; if I recall correctly, New XKit's NotificationBlock could be invoked on reblogs, and we may as well process both the post id and post root id.

[stale]

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