Added blob holder operations when changing thick thread avatars.

We want to:

• Always remove a holder if the old avatar was image
• Establish holder if the new avatar is image. We want it only for inbound ops. For outbound, the holder is already established during image upload.

Depends on D13411

Created a DM thread with two devices.

• Set avatar to image on device A. Only device B generated op to establish holder.
• Changed avatar to another image. Both devices generated op to remove avatar. Only inbound device established holder.
• Reset avatar to emoji. Both devices removed their avatars