don't prevent the html pages to load a favicon, and provide one by default

for issue #186 by morki, thanks for reporting and providing sample favicons.

generated by the mentioned generator at favicon.io, with the ubuntu font and a
fuchsia-like color.

the favicon is served for listeners/domains that have the
admin/account/webmail/webapi endpoints enabled, i.e. user-facing. the mta-sts,
autoconfig, etc urls don't serve the favicon.

admins can create webhandler routes to serve another favicon. these webhandler
routes are evaluted before the favicon route (a "service handler").

webadmin/admin.html

@@ -4,7 +4,6 @@
 		<title>Mox Admin</title>
 		<meta charset="utf-8" />
 		<meta name="viewport" content="width=device-width, initial-scale=1" />
-		<link rel="icon" href="noNeedlessFaviconRequestsPlease:" />
 		<style>
 body, html { padding: 1em; font-size: 16px; }
 * { font-size: inherit; font-family: ubuntu, lato, sans-serif; margin: 0; padding: 0; box-sizing: border-box; }