Add a command to find users dropped by follow-up campaigns.

[toolness]

Note: this PR isn't intended for merging, I'm going to just run it locally and then close this PR to ensure that the code is recorded somewhere.

This adds a command that outputs a CSV of RapidPro contacts who never received follow-up campaign SMS messages due to the situation described in #902.

[toolness]

Relatedly, here's a command to follow up with dropped users:

import datetime
from django.conf import settings
from django.core.management.base import CommandError, BaseCommand
from temba_client.v2 import TembaClient
from temba_client.utils import parse_iso8601, format_iso8601

from rapidpro.rapidpro_util import get_group, get_field


class Command(BaseCommand):
    def add_arguments(self, parser):
        parser.add_argument('--dry-run', help="don't actually update records",
                            action='store_true')
        parser.add_argument('--only-contact-uuid', help="limit migration to one contact UUID")

    def follow_up_with_contacts(
        self, client: TembaClient, group_name: str, field_key: str,
        only_contact_uuid: str, dry_run: bool, date_diff: datetime.timedelta
    ):
        group = get_group(client, group_name)
        assert get_field(client, field_key) is not None
        now = format_iso8601(datetime.datetime.now())
        prefix = "[DRY RUN] " if dry_run else ""
        print(f"{prefix}Finding contacts in RapidPro group '{group_name}' whose {field_key} "
              f"exceeds their creation date by {date_diff} and replacing it with "
              f"{now}.")
        kwargs = {'group': group}
        if only_contact_uuid:
            kwargs['uuid'] = only_contact_uuid
        contacts = client.get_contacts(**kwargs).iterfetches(retry_on_rate_exceed=True)
        for contact_batch in contacts:
            for contact in contact_batch:
                field_value = parse_iso8601(contact.fields[field_key])
                assert isinstance(field_value, datetime.datetime)
                assert isinstance(contact.created_on, datetime.datetime)
                if field_value - contact.created_on > date_diff:
                    update = {field_key: now}
                    print(f"{prefix}Updating {repr(contact.name)} fields {update}")
                    if not dry_run:
                        client.update_contact(contact, fields={
                            **contact.fields,
                            **update
                        })

    def handle(self, *args, **options):
        if not settings.RAPIDPRO_API_TOKEN:
            raise CommandError("RAPIDPRO_API_TOKEN must be configured.")
        client = TembaClient(settings.RAPIDPRO_HOSTNAME, settings.RAPIDPRO_API_TOKEN)
        only_contact_uuid: str = options['only_contact_uuid'] or ""
        dry_run: bool = options['dry_run']

        self.follow_up_with_contacts(
            client,
            'DHCR Requested Rental History',
            'date_of_dhcr_req_rental_history',
            only_contact_uuid,
            dry_run,
            datetime.timedelta(days=19)
        )