[fix bug 1009892] Prepare /whatsnew to be displayed for updates from pre-29 versions

So the new behavior for `/whatsnew` going onwards is going to be:

* default: no-tour (just the post tour web page)
* if ?oldversion < 29, show the tour.

This applies to versions 29 through to 31 (after which /whatsnew is due to be turned off in product).

This means folks upgrading from pre-29 versions will still see the tour, even if they go straight to Firefox 30 or 31. For those who have already seen the tour, we will still show them the post-tour web page (but they should not get the tour again).