Strip restricted and incomplete handles from the "Mentions" tab on Maniphest tasks

Summary:
Ref T8345. See T8345#201048 for discussion.

This rule (don't show mentions of or from restricted objects) is more consistent with how we render mentions in the timeline and I think generally a better behavior.

Test Plan:

• Mentioned a task on a public task and a private task.
• Privileged user (foreground) sees both.
• Public user (background) sees only the public mention.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T8345

Differential Revision: https://secure.phabricator.com/D16900