Remove deleted_at (Postgres) and _deleted_at (Mongo)

[jnm]

We are preparing to remove deleted_at (Postgres) and _deleted_at (Mongo) for performance reasons. We will stop setting these attributes in the legacy view and delete the submissions outright, as was always done for DELETE HTTP requests to the API. We will also include a migration that deletes all rows in Postgres' logger_instance that have any non-null value for deleted_at, and likewise remove any documents in the Mongo instances collection that have _deleted_at set.

django-reversion will not be changed. logger_instancehistory rows will be removed when their corresponding rows in logger_instance are removed. This matches the existing behavior when deleting with the API, but it will apply retroactively to all instances / submissions that were deleted with the legacy view, i.e. instances that have been marked as deleted but still exist in the database.

Originally posted by @jnm in #659 (comment)

ℹ️ Once this is done, we can remove _deleted_at from queries in KPI as well.

[jnm]

I'm purging most of these on production now to avoid a long, downtime-inducing migration later:

import sys
import time
batch_size = 25
failures = []
byebye = list(Instance.objects.exclude(deleted_at=None).values_list('pk', flat=True))
while True:
    pk_batch = byebye[:batch_size]
    if not pk_batch:
        break
    del_candidates = Instance.objects.filter(pk__in=pk_batch)
    # paranoia
    del_candidates = del_candidates.exclude(deleted_at=None)
    try:
        del_candidates.delete()
    except Exception as e:
        for dc in del_candidates:
            try:
                dc.delete()
            except Exception as e:
                sys.stdout.write('!')
                sys.stdout.flush()
                failures.append(dc.pk)
        sys.stdout.write('\n')
    del byebye[:batch_size]
    sys.stdout.write('\r{} remain'.format(len(byebye)))
    sys.stdout.flush()
    time.sleep(1)

The hacky excepts are to cope with #697