[keyserver] Update deleteThread and deleteInaccessibleThreads to handle thread image avatars

Summary:
I looked through all of the DELETE queries in keyserver to see where we're handling deletion of threads. I found two queries (deleteThread and deleteInaccessibleThreads) that needed to be updated to handle deletion of thread avatar (AKA upload with container=threadID)

Depends on D7376

Test Plan:

1. Set image avatar for thread.
2. Observe that container is set correctly in uploads table.
3. Delete thread.
4. Observe that row in uploads table has been removed.

Reviewers: ashoat, ginsu

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D7377