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

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 (