Polling interval field CSS update

As described in ticket: https://issues.apache.org/jira/browse/COUCHDB-2371

I also changed the slider event from onChange to onInput which continually fires when the user drags the slider. It results in more code being called but it has a much better feel, I think: i.e. the # seconds updates during dragging, not just afterwards. This was probably what was originally intended: Chrome changed the onChange event behaviour a few months back to only fire after the drag is completed.
Doh, old habits die hard. I might add a Grunt task to check for this...