Add contentnode deduplication to import from other channel search

[vkWeb]

In #3399 we removed deduplication of content nodes from import from other channel search due to #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.

[vkWeb]

@bjester sir should we add this to Q3 milestone, could be fixed with work on studio's recommendation system?

[bjester]

We can add it to that milestone, but it won't intersect at all with the recommendation system

[vkWeb]

Okay.

Yes sir, it won't intersect with the recommendation system but since we'll be focusing on search & recommendations so thought to put this in that milestone.