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.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T8980

Differential Revision: https://secure.phabricator.com/D14878