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

pkg/asinfo/example_test.go

@@ -21,9 +21,16 @@ func ExampleGetDescription() {
 }
 
 func ExampleSearch() {
-	results := asinfo.Search("MIT-GATEWAY")
+	const limit = 10
+
+	results, total := asinfo.Search("mit-gateway", limit)
+
+	fmt.Printf("%d of %d matches\n", len(results), total)
+
 	for _, info := range results {
 		fmt.Printf("AS%d: %s - %s\n", info.ASN, info.Handle, info.Description)
 	}
-	// Output: AS3: MIT-GATEWAYS - Massachusetts Institute of Technology
+	// Output:
+	// 1 of 1 matches
+	// AS3: MIT-GATEWAYS - Massachusetts Institute of Technology
 }