Strip restricted and incomplete handles from the "Mentions" tab on Maniphest tasks
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.
- Mentioned a task on a public task and a private task.
- Privileged user (foreground) sees both.
- Public user (background) sees only the public mention.
Reviewed By: chad
Maniphest Tasks: T8345
Differential Revision: https://secure.phabricator.com/D16900