Added private thread icon, also made it so the thread visiblity section in thread settings says "private" near the new icon

Summary: Added private thread icon, also made it so the thread visiblity section in thread says "private" next to the new icon.

Test Plan: Tested indivdually and in combination with the other code required to create/use private threads. Tested by creating new accounts, clicking on the private thread, going into settings of the private thread. Sending messages to the private thread. And also making sure the previous thread types still work by creating other accounts and messaging each other.

Reviewers: ashoat, palys-swm

Reviewed By: ashoat

Subscribers: KatPo, zrebcu411, Adrian, atul

Differential Revision: https://phabricator.ashoat.com/D624