[fix bug 995467] Update post tour landing page (8 May)

* Glow CTA is now is hidden by default on the post-tour landing page.
* If a user is already using Sync, the Sync CTA is hidden and the Glow CTA is shown as replacement.
* If JS is disabled, we make no assumptions and show both CTA's.
* Applies to both `/whatsnew` and `/firstrun`.

Note: you can sign in and out of Firefox Sync in preferences to test this change.