[web] Support setting image avatars for thick threads

Summary:
Updated avatar upload hook on web to make keyserver upload optional. Disabled upload for thick threads. Forced Blob uploads for non-keyserver uploads, regardless of the flag.
This follows chagnes from D13280.

Depends on D13280, D13285

Test Plan:

• Successfully set image avatar for DM thread, with keyserver killed.
• Keyserver avatars work as before

Reviewers: kamil, tomek

Reviewed By: tomek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D13286