Add stop-at-bottom functionality

Detach all methods from sticky reference so they
can be attached to different objects.

Split sticky into stickAtTop and stickAtBottom and
make new versions of all methods and properties
specific to stickAtBottom.

Add CSS for stickAtBottom and call on load

app/assets/javascripts/main.js

@@ -1,6 +1,7 @@
 $(() => $("time.timeago").timeago());
 
 $(() => GOVUK.stickAtTopWhenScrolling.init());
+$(() => GOVUK.stickAtBottomWhenScrolling.init());
 
 var showHideContent = new GOVUK.ShowHideContent();
 showHideContent.init();