Make event detail view more user-friendly for imported events

Summary:
Ref T10747. When viewing an imported event:

• Make it more clear that it is imported and where it is from.
• Add some explicit "this is imported" help.

Test Plan: Viewed imported and normal events.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10747

Differential Revision: https://secure.phabricator.com/D16727