Implement safe find/replace for user-provided links Extract an interface out of RegexFindReplace for replacement with arbitrary types of sanitization or other munging. Implement a LinkFindReplace that knows how to sanitize links with SafeHtmlBuilder. Push the replacement into the FindReplace interface so implementations can do sanitization _after_ regular expression replacement. Change-Id: I46383abd1921a11b4c90b80b2022051fc0c8e13c