Checkout does not scroll to top on SPA load

[sirbrillig]

Calypso is a Single Page App (SPA) so when navigating between pages, you're actually staying on the same page (as far as the browser knows) most of the time; the code just updates the URL and the page contents. As a result, when you scroll down on one page and then navigate to another which also has a scrollbar, sometimes that second page is scrolled down too (the scroll state actually just doesn't change since you're still actually on the same page).

This can be fixed by explicitly scrolling to the top when a page's top-level component renders. For example as it was done here:

#45933

Checkout does not do this and it should.

To reproduce:

• Make a new site incognito, free. Viewport is laptop size-ish.
• Navigate to Jetpack Stats
• Scroll to the bottom of the screen and click the upgrade CTA.
• Notice that checkout is scrolled part-way down the page.