Rework CWs and long post handling, simplify code and user interface

[ariasuni]

Pitch

Current idea

• Make CW in posts prettier (use purple ✨)
• Change CW from title + button, to one button with same width as post for accessibility
• Rework CW field in compose field (maybe just remove the warning borders?)
• (maybe?) Remove icons in CW button (redundant with status icons)
• Remove post collapsing:
  • for notifications, use grouped notifications (hopefully soon)
  • for long posts, use upstream logic and same visual/accessibility improvements as CW
• In boost modals, auto-deploy the post (instead of hiding the post as currently done, or removing CW like upstream)

Motivation

Current CW design is starting to show its age, but a lot of people don’t like upstream’s design. We need something better and prettier.

[cairobraga]

(considering how absolutely horrible the new upstream UI is, this is needed more than ever.)