Delete Term Images from Object Storage When Deleting Terms & Studysets

[ehanahamed]

Terms get deleted when their studysets are deleted since they have the studyset_id column with on delete cascade. However, we need to also make sure term images in object storage are deleted when deleting terms or whole studysets.

Also see quizfreely/idb-api-layer issue #4

[ehanahamed]

Instead of getting keys right before deleting terms and then deleting from object storage using those keys, there's now a term_images table with an object_key column and term_id column. When a term is deleted (or a studyset is deleted and it automatically deletes the terms inside of it too), any rows in term_images for those term IDs will have term_id become null, and there's a cron job that deletes all old term_images from object storage for rows that have a null term_id.