Add index customize animation.

- Add delay to overview intro animation.

- Small UI fixes/tweaks.
Merge remote-tracking branch 'upstream/bug-975156-fx-desktop-pages' into bug-975156-fx-desktop-pages

Conflicts:
	bedrock/firefox/templates/firefox/desktop/index.html
	media/css/firefox/desktop/index.less
	media/js/firefox/desktop/index.js