[keyserver] Update thread avatar assigner to check new user_container column

Summary:
We should not assign an upload as a thread avatar if it's already assigned as a user avatar.

Depends on D11991

Test Plan:
I tested the whole stack with the following steps:

1. Ran the migration on a backup of my production keyserver database from September 2023. Confirm that the uploads table has the new column, that it has an index, and that it's populated for all cases where the old column matched a user. Confirm that only 32 rows were updated, which matched the number of users with avatars in a prior query
2. Create a brand new database. Confirm that the Confirm that the uploads table has the new column, and that it has an index. Make sure DESCRIBE uploads; matches what it looked like in step 1
3. Run the migration on my local multi-keyserver testing database. Make sure I'm still able to log in with an existing user. Confirm I can change the existing user's avatar to a photo and that there are no errors in the logs. Confirm that the avatar is still there if I log out and back in
4. On the same database, try creating a new user with a photo avatar and confirm that it works, and that there are no errors in the logs. Confirm that the avatar is still there if I log out and back in
5. Ran git grep container in the keyserver folder and reviewed every result to consider whether it needs to be updated

Reviewers: tomek

Reviewed By: tomek

Differential Revision: https://phab.comm.dev/D11992