Mechanism for Boosts to be requested and integrated into an existing feed

This will be an implementation/extension of the Feed class that was introduced for Boost Campaign refactoring work.

NB: Needs that work to be cherry-picked into master for this work to be based off.

• Remove m-featured-content and dependency on FeaturedContentService
• Place in boosts from backend api

Dev notes

• Core/Feeds/Top/Manager
• Boosts to be placed inside of FeedSyncEntity
• Benefit of this is changing boosts slots from single backend location