for WebRedirect, don't "match" when the destination URL has the same scheme,host,path, for doing http -> https redirects without loops

you can already get most http to https redirects through DontRedirectPlainHTTP
in WebHandler, but that needs handlers for all paths.

now you can just set up a redirect for a domain and all its path to baseurl
https://domain (leaving other webdirect fields empty). when the request comes
in with plain http, the redirect to https is done. that next request will also
evaluate the same redirect rule. but it will not cause a match because it would
redirect to the same scheme,host,path. so next webhandlers get a chance to
serve.

also clarify in webhandlers docs that also account & admin built-in handlers
run first.

related to issue #16

http/webserver.go

@@ -341,6 +341,19 @@ func HandleRedirect(h *config.WebRedirect, w http.ResponseWriter, r *http.Reques
 	if h.StatusCode != 0 {
 		code = h.StatusCode
 	}
+
+	// If we would be redirecting to the same scheme,host,path, we would get here again
+	// causing a redirect loop. Instead, this causes this redirect to not match,
+	// allowing to try the next WebHandler. This can be used to redirect all plain http
+	// requests to https.
+	reqscheme := "http"
+	if r.TLS != nil {
+		reqscheme = "https"
+	}
+	if reqscheme == u.Scheme && r.Host == u.Host && r.URL.Path == u.Path {
+		return false
+	}
+
 	http.Redirect(w, r, u.String(), code)
 	return true
 }