Merge branch '4.5' into 4.6

FIX Preview email link now handles cases where it's loaded in the bro…

…wser, requested via AJAX and used in a trait or a page context (silverstripe#887)

FIX Preview email link now handles cases where it's loaded in the browser, requested via AJAX and used in a trait or a page context

Release 5.4.0

Merge branch '5.2' into 5.3

Release 5.3.1