Add timeout to http server

[jholdstock]

HTTP server is initialized without a timeout. This is highlighted by the linter as a security issue.

G114: Use of net/http serve function that has no support for setting timeouts (gosec)

dcrvotingweb/main.go

Lines 449 to 453 in 39589dc

	err = http.ListenAndServe(cfg.Listen, nil)
	if err != nil {
	log.Printf("Failed to bind http server: %v", err)
	cancel()
	}