Add basic structure for unified prologue content views

[frosty]

Ref #15056

This PR implements a basic structure for adding content views to the new unified prologue. It simply adds some code to add ContentView UIView subclasses to the main carousel for each page, with a skeleton implementation of the intro screen's content view.

Note that the contents of the intro screen isn't final, just a placeholder – but this PR is just to allow us to start working on the other screens in parallel.

To test

PR submission checklist:

• I have considered adding unit tests where possible.
• I have considered adding accessibility improvements for my changes.
• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

[peril-wordpress-mobile]

You can trigger an installable build for these changes by visiting CircleCI here.

[peril-wordpress-mobile]

You can trigger optional UI/connected tests for these changes by visiting CircleCI here.

[Gio2018]

Hey @frosty , thank you for this!

I know it's not final and the feature flag is disabled, but wanted to point out that the views/constraints will need to be adjusted to fit the iPad too. Here's what it looks as of now, compared to what we have in the designs

design	actual

Approving this one so we can move forward and make the changes.

[frosty]

Thanks! Yep, definitely needs lots of adjustment for iPad!