Update Reader Comments with the new design

[dvdchr]

Refs pbArwn-2ks-p2

Reuse the existing components and logic in CommentDetailViewController for the new designs in ReaderCommentsViewController, particularly the header cell, content cell, and the moderation logic.

Designs from Zeplin:

Tasks 🎯

• Add feature flag for the updates on Reader Comments. Addressed in Comments: Extract comment header cell configuration #17480
• Comment header cell
  • Update the header cell so it can show/hide disclosure indicator. Addressed in Comments: Extract comment header cell configuration #17480
  • Reader Comments: Display the new comment header cell #17487
  • Reader Comments: Show disclosure when pushing detail is allowed #17517
• Reader Comments: Reuse the new comment content cell #17476
• Show an overflow dropdown menu for administrator roles.
  • Implement the overflow dropdown menu. Addressed in Reader Comments: Overflow dropdown menu #17601.
  • Reader Comments: Add fallback implementation for overflow menu on iOS 13 #17622
  • Present a share sheet when the Share menu is tapped. Addressed in Reader Comments: Overflow dropdown menu #17601.
  • Reader Comments: Implement edit action on the overflow menu #17623
  • Reader Comments: Implement moderation actions #17629
• Review accessibility.
• Comments: Add tracking for comment sharing intent #17658
• Reader Comments: Enable the new comment threads. #17659