Add an option to disable the ad preview on the node edit form

When using HTML ads a preview of the ad is displayed above the html text area.

When using some doubleclick ad javascript in the html field this preview was causing huge problems.

Specifically, just as the page loaded, the status bar said "stopped" and I got a white screen and the page just tries to load forever.

It's a bit weird because the ad works normally when viewing outside of the node edit screen.

To easily get around this I just added an option to set whether or not this preview is displayed.

Here is a patch.