Allow host apps to customize the background of the login prologue

[ctarda]

Closes #538
Ref: woocommerce/woocommerce-ios#3319

LoginPrologueViewController sets its background color to WordPressAuthenticatorUnifiedStyle.unifiedBackgroundColor.

For WooCommerce, we would need to be able to set the background color of the area where the action buttons sit extend to be as wide as the screen.

Changes

• Added a new parameter to WordPressAuthenticatorUnifiedStyle with a value that defaults to viewControllerBackgroundColor.
• Add some logic to LoginPrologueViewController to assign this new color and hide the blurred view. I don't love how this is implemented, but I am not sure there is a much better way to do it.

How to test

There are no changes to the API, but just in case I submitted a draft PR to WordPress that should prove that nothing breaks: wordpress-mobile/WordPress-iOS#15464 The best way to test this is:

• Checkout the PR in WooCommerce that uses these changes: Fix background color of Prologue buttons in landscape on iPad woocommerce/woocommerce-ios#3320
• Checkout also the PR in WordPress that proves that these changes do not break anything: Point WPauthenticator to the relevant branch WordPress-iOS#15464