Improve the deleted puzzles interaction

[rgossiaux]

We might want to just mark them with a flag rather than actually deleting them from the db, to make it easier to restore them in case of an accident. Right now you have to create a new puzzle and then edit its sheet field, and this flow is pretty awkward/bad (eg timestamps messed up, chat channel associations lost)

[rgossiaux]

This also causes cache problems with get_puzzle_pk_from_drive_item if we ever change sheet IDs manually since we never clear the cache (maybe should track separately tbh)

[rawxfish]

Could potentially use something like this: https://pypi.org/project/django-soft-delete/

#548 also tangentially related to this.