Bug 16959 Rename local -> floating date/time for clarity

"Local time" is a confusing term and could easily refer to global time
in my local time zone. Renaming to i18n suggested "floating" time to
more accurately reflect the concept. Editor's Note: references to
"local" date and time in sections of the spec forked from WHATWG are
left untouched to avoid unnecessary merge conflicts. Changes are made
elsewhere to help us more easily find and fix broken links due to future
cherry-picks.