[lib] Fix resetting thick thread avatars

Summary:
Addresses ENG-9208.
The case when avatar was reset for DM thread, wasn't supported in the thread-actions.js and later in the DM op.

Investigated how this is handled on keyserver and added appropriate logic here.

Test Plan: Confirmed that clicking the "Reset avatar" button in thread settings menu actually resets the avatar.

Reviewers: kamil, tomek, inka

Reviewed By: tomek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D13283