fix prefix search first seen

[lanrat]

prefix search: populate first_seen on first_imports, here and elsewhere
ex: https://dns.coffee/search/prefix/active/NIC

Can use first_import as first_seen for zones because all the new TLDs where created after I started indexing the root. (double check that the logic fully applies).

[lanrat]

The issue here is to fix the blank or missing "First Seen" dates.

In the DB schema, I leave first_seen null if the record was present on the first import for the given zone, and the zone was not newly created (in DNS). However, for only the purposes of this search prefix page, we can populate the missing "First Seen" fields with the dates of the first import for the zone.