[keyserver] Delete invite links when deleting threads

Summary:
We should delete invite links when we delete threads. This diff makes sure we delete both thread-specific and community-specific invite links when the thread/community is deleted.

Depends on D13732

Test Plan:

1. Create a community, enable invite link. Confirm it exists in MariaDB and DynamoDB. Delete it and confirm that it's gone from both
2. Same as above, but for a thread-specific invite link, which was created using this script: https://gist.github.com/Ashoat/0c3815442bf6427ecd7d8a3dbdb4e5dd
3. Same as above, but for a community that has a community-specific invite link, and a thread inside that community that has a thread-specific invite link. Delete the community and confirm that both invite links are gone from MariaDB and DynamoDB

Reviewers: tomek

Reviewed By: tomek

Differential Revision: https://phab.comm.dev/D13733