Subsequent commits will add "on_delete=models.RESTRICT"
relationships, which will result in the Attachment
objects being deleted after Realm has been deleted from
the database.
In order to handle this, we update
get_realm_used_upload_space_cache_key function to accept
realm_id as parameter instead of realm object, so that
the code for flushing the cache works even after the
realm is deleted. This change is fine because eventually
only realm_id is used by this function and there is no
need of the complete realm object.
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