Homestyx hydra

R1:97cd7a98b1e3

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.

{F1929485}

Reviewers: chad…
Repository: R1 hydra
Commit Date: Nov 18 2016