Move common appearance code out of PaywallViewController into AppearanceConstants.
Introduce code for ternary view controllers with Header / Description / Button for first screen and confirmation screens.
Next AIs:
* wire up first page view controller
* implement and wire up confirmation screens.
Moving all categories into one file. Creating new helper category for UILabel / UIButton to help with messages / text-like buttons.
a lot of code deduplication in this change.