Add contentnode deduplication to import from other channel search

[vkWeb]

In https://github.com/learningequality/studio/pull/3399 we removed deduplication of content nodes from import from other channel search due to https://github.com/learningequality/studio/issues/1055 a long standing bug on studio where in two contentnodes with different content can have the same content_id.

We need to re-think our approach on how we treat equality of two contentnodes. Once that's done, we can implement deduplication.