Fix "Full Site" behaviour

[sminnee]

From http://open.silverstripe.org/ticket/5953

Currently the "Full Site" link doesn't work when redirecting mobile devices to the mobile domain automatically.

A cookie is set, but it doesn't appear to be shared between the full site and the mobile site. Perhaps the domain argument needs to be set for setcookie()?

Would this work if the mobile site was "mysite.mobi" instead of a subdomain "m.mysite.com"?

Automatic redirection behaviour:

When first redirected to mobile site on a mobile device, set a cookie "onMobile" true and another "onFullSite" false to keep track of the state
When full site requested, set another cookie "onFullSite" true and set "onMobile" to false, and check the cookie states to ensure automatic redirects to the mobile site don't occur any longer
When mobile site accessed again after viewing the full site, set "onMobile" true and "onFullSite" false, re-enabling automatic redirects to the mobile site

[mmichelli]

A suggested change/fix: #31