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).

• Viewed people, task, revision, commit and project hovercards.