* Copy FeedViewPost into FeedSliceItem
* Explicitly construct feed slice items by copying known fields
* Type rootItem as FeedViewPost for now
Mergefeed logic relies on that.
* Unify reason and __source for slice items
* Move feedContext out of FeedSliceItem
* Remove slice.isFlattenedReply
* Remove unnused slice.ts
* Inline slice.isFullThread
* Refactor condition for clarity
* Extract slice.includesThreadRoot
* Encapsulate more usages of slice.rootItem into slice
* Rename slice.rootItem so semi-private slice._feedPost
* Move reason into slice
* Simplify slice ctor argument
* Reorder getters to reduce diff
* Make feedContext a getter to reduce diff
ac1538baad