Removed unnecessary timestamp null check

Because the XHR date header is being used in a calculation the end
result should always be NaN if the date is null.
Rework nag bar

Nag bar now displays under the following circumstances:
- The user is scrolled down the page and a new entry is processed.
- The user is scrolled down the page and an update is processed that
would change content above the bottom of the viewport.

On new entry, clicking on the nag bar will now smooth-scroll to the top.
Fix IE8 nag bar background

IE8 doesn't support RGBA so add opacity filter for IE fallback.