Make thread-visiblity icon use thread-icon

Summary: Made it so thread-visiblity uses thread-icon.react.js instead of its own icon.

Test Plan: Checked in thread settings to see if icon still worked. Also checked private threads before and after updates to make sure they still work. And also made sure normal threads still work

Reviewers: ashoat, palys-swm

Reviewed By: ashoat

Subscribers: KatPo, zrebcu411, Adrian, atul

Differential Revision: https://phabricator.ashoat.com/D626