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/server/server.go

@@ -1,4 +1,8 @@
-// Package server provides HTTP endpoints for status monitoring and statistics
+// Package server serves RouteWatch's web pages and JSON API:
+//   - the home page, the status figures and the health check
+//   - the AS, prefix and prefix length pages, and the JSON answers behind them
+//   - the IP lookups
+//   - the AS name search
 package server
 
 import (