Upgrade message not shown in about:tor

Noticed this when upgrading my test profile to 15.0, the initial about:tor page did not show the upgrade message. But a second about:tor page did.

I don't remember this happening in my regular profile, but my new tab is set to about:blank there.

So I think this is likely because about:tor is pre-loaded when it is set as the new tab page. And this may be before torbrowser.post_update.shouldNotify is set.

I think we could resolve this by only clearing the torbrowser.post_update.shouldNotify preference after about:tor is actually shown.