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").
