Now that we're enabling the feature in the UI, we should set
these to the planned long-term defaults for these settings.
Also, this commit cleans up the '0476' and '0477' migration
files related to user_topic policies.
'0476' sets 'null=True'
'0477' is noop
'0482' sets the default values and performs backfilling.
Co-authored-by: Tim Abbott <tabbott@zulip.com>
