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/ristypes/ris.go

@@ -39,12 +39,16 @@ type ASPath struct {
 	Set []int
 }
 
+// NoOrigin is the origin AS of a route whose origin is not known. AS 0 is
+// reserved, so no AS has this number.
+const NoOrigin = 0
+
 // Origin returns the AS that originated the route: the last AS of the
-// sequence, or 0 (none) when the path ends in a set of several members or is
+// sequence, or NoOrigin when the path ends in a set of several members or is
 // empty.
 func (p *ASPath) Origin() int {
 	if len(p.Set) > 0 || len(p.Sequence) == 0 {
-		return 0
+		return NoOrigin
 	}
 
 	return p.Sequence[len(p.Sequence)-1]