replace /billing link which leads to 404

[michaelrichey]

Two KG pages had the link:
https://www.wanikani.com/account/subscription/billing

Which leads to 404

• replace 404 link with link to subscription page
• update copy to reflect how to accurately perform tasks
• update screenshots

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[michaelrichey]

@rachelcgrant This is ready for review.
In grabbing new screenshots to replace the old billing page images, I noticed that the prompt for updating details changes depending on whether or not you have an active subscription. It's either:

• manage details
  or
• update details

So I included both screens and copy for both. However, this may not matter that much. I was trying to be accurate in my description, but let me know if it's confusing instead. It may be enough to just say "click the button to manage your payment or subscription details" and not have a screenshot.