[NT-1256] iOS 12 pledge screen scroll view insets bug

[ifbarrera]

📲 What

Fixes a bug in iOS 12 where presenting a VC from the pledge screen caused the scroll view insets to be reset to zero, which caused the pledge button container to overlap some content near the bottom of the screen.

🤔 Why/How

This was a strange one. For some reason on iOS 12.4, presenting a VC on top of a scroll view with its contentInsetAdjustmentBehavior set to automatic (the default), caused the content insets to abruptly be reset back to zero when the presented VC is dismissed. Strangely, examining the contentInsets after the dismissal showed that the adjustedContentInsets were still correctly set, but visually the scroll view appeared to have an adjustedContentInset of .zero. Setting the contentInsetAdjustmentBehavior on the rootScrollView to .zero seemed to fix the issue, but it caused a strange side effect where the scroll view no longer bounced properly.

As a workaround, this PR pins the rootScrollView's bottom to the top of the pledgeCTAContainerView so we don't rely on the contentInsets being correct.

👀 See

Trello, screenshots, external resources?

After 🦋	Before 🐛

✅ Acceptance criteria

• on an iOS 12 device, navigate to the pledge screen and check that the view scrolls up fully without obscuring content. Then, tap the "New Payment Method" button and close the view. Check that the view scrolls up fully without obscuring content.
• on an iOS 13 device, navigate to the pledge screen and check that the view scrolls up fully without obscuring content. Then, tap the "New Payment Method" button and close the view. Check that the view scrolls up fully without obscuring content.

[justinswart]

Thanks for fixing!

[justinswart]

This is already set in the lazy var property.