[lib] Support image avatars in DM thread-action

Summary:
Addresses 2. in ENG-9102.

Updated DM part of thread-actions.js to support new avatar type introduced in D13284

Depends on D13283, D13284

Test Plan: Tested in the next diff.

Reviewers: kamil, tomek, inka

Reviewed By: tomek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D13285