Checkout process should better handle slower speeds

[miguelsolorio]

Noticed this when I was on a slow connection, clicking on "Checkout" instantly disables the button, but while the process continues the button re-enables but the UI hasn't changes to reflect the change and I'm able to click on "Checkout" again.

Repro

1. Throttle speed
2. Open PR
3. Click on "Checkout"
4. See "Checkout" button get re-enabled after a few seconds while page is still processing (see progress bar in sidebar)

Expected

• Button should only be re-enabled once UI has updated