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>, which
searches the AS list built into asinfo, and lists the matches as links
to their AS pages. An empty q answers 400. asinfo.Search now ignores
case and returns its matches in AS number order.

A match can be an AS that the database does not hold yet. Its AS page
and /api/v1/as/{asn} now answer with its handle and description from
asinfo and no prefixes, instead of 404. Empty prefix lists in that JSON
answer are [] rather than null.

Model: opus-5-5

pkg/asinfo/example_test.go

@@ -21,7 +21,7 @@ func ExampleGetDescription() {
 }
 
 func ExampleSearch() {
-	results := asinfo.Search("MIT-GATEWAY")
+	results := asinfo.Search("mit-gateway")
 	for _, info := range results {
 		fmt.Printf("AS%d: %s - %s\n", info.ASN, info.Handle, info.Description)
 	}