Make the home page's AS name search work (closes #151)

The AS Name Search card on the home page said the search was coming soon.
It now asks the new GET /api/v1/search/as?q=<text> and lists the matches
as links to their AS pages. asinfo.Search ignores case, returns the first
matches up to a limit in AS number order, and counts all matches. An
empty q answers 400.

When the database has no record of an AS, its page and /api/v1/as/{asn}
take its handle and description from asinfo and still list its prefixes.
They answer 404 only for an AS with no record, no asinfo entry and no
prefix. AS 0 means no known origin: it always answers 404, and the search
never lists it.

Model: opus-5-5

internal/templates/as_detail.html

@@ -233,10 +233,12 @@
                 <div class="info-label">Peer ASNs</div>
                 <div class="info-value">{{.PeerCount}}</div>
             </div>
+            {{if not .ASN.FirstSeen.IsZero}}
             <div class="info-card">
                 <div class="info-label">First Seen</div>
                 <div class="info-value">{{.ASN.FirstSeen.Format "2006-01-02"}}</div>
             </div>
+            {{end}}
         </div>
 
         {{if .IPv4Prefixes}}