[NT-320] Improve experience on SheetOverlay for vertically compact environments

[ifbarrera]

📲 What

This PR makes some improvements to our custom SheetOverlayViewController to better support vertically compact environments.

This PR also updates the iPad presentation style of ManagePledgeViewController to .pageSheet so it's more consistent with how the PledgeViewController is presented on iPad.

🤔 Why

On landscape mode on some devices, the shipping location selector on the pledge screen was very difficult to use, because it was presented inside the SheetOverlayViewController with a very large offset. This created a very frustrating user experience.

🛠 How

The root issue is that on landscape mode (in vertically compact environments), the offset on the SheetOverlayViewController is too large, even though it works for portrait or vertically regular environments. To get around this, we override traitCollectionDidChange on SheetOverlayViewController to check if the new trait collection is vertically compact. If it is, we simply update the offset to a more reasonable number for that trait collection: the height of the navigation bar, or a hard-coded similar value in the case where there is no navigation bar. If the user then rotates back to a vertically regular environment, the originally defined offset is used.

👀 See

Trello, screenshots, external resources?

After 🦋	Before 🐛

iPhone X

iPad

Manage Pledge presented using Page Sheet on iPad

♿️ Accessibility

N/A

🏎 Performance

N/A

✅ Acceptance criteria

• Navigate to the pledge screen and tap the shipping location button to display a list of shipping locations. Then, transition to landscape mode. You should see a smaller offset in landscape mode, and returning to portrait mode should show the original offset.

[cdolm92]

ACs met, Tested on:

• iPhone XS simulator
• iPad Air, 12.0