API CHANGE: Added two arguments to HTMLText#Summary. Minimal impact since previously any usage of this function threw an error.
ENHANCEMENT: Add HTMLText#FirstSentence based on new HTMLText#Summary
git-svn-id: svn://svn.silverstripe.com/silverstripe/open/modules/sapphire/branches/2.3@78618 467b73ca-7a2a-4603-9d3b-597d59a354a9