R1:95c2b03fc8e5

Distinguish between invalid/broken handles and filtered handles

Summary:
Ref T603. Currently, we render handles the user doesn't have permission to see in a manner identical to handles that don't exist. This is confusing, and not required by policies (which restrict content, but permit knowledge that an object exists).

Instead, render them in different styles. Bad/invalid objects look like:

Unknown Object (Task)

Restricted objects look like:

[o] Restricted Task

...where `[o]` is the padlock icon.