R1:bdc517485cb2

Modernize Hovercard implementation

Summary:
Ref T8980. Move away from events to EngineExtensions.

This also simplifies hovercards a bit:

- Removes tasks from revision cards.
- Removes blockers/blocked from task cards.
- Removes "Send Message" from user cards.

These mostly felt cluttery to me. Open to arguments to retain them. I think we can make better use of the space, though (e.g., flags, projects + board columns).

Test Plan:
- Viewed people, task, revision, commit and project hovercards.