Don't load attached files for profile images

Summary:
Ref T8478. I think the cycle is:

• Conpherence Thread > Loads handle for participant > loads file for profile image > loads attached files to check visibility > loads conpherence thread > ...

So, specifically, someone attached their profile image to a thread or message somewhere.

This breaks the cycle by stopping the attached-files visibility check from happening, since we don't need it. This seemed like the easiest link in the chain to break.

Ideally, I think the longer-term and more complete fix here is to stop Conpherence from requiring handles in order to load thread handles (and, generally, having a "handles must not load other handles" rule), but that's not trivial and might not be especially practical.

Test Plan: Will test in production.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T8478

Differential Revision: https://secure.phabricator.com/D13216