Add Reader screen for unified prologue carousel

[frosty]

Ref #15056. This PR adds the Reader screen to the prologue carousel.

iPhone	iPad

To test

• Build, run and log out if necessary
• Verify that the Reader (last) page of the login prologue is consistent with the designs (see issue Prologue Carousel - Project Issue #15056; see also example screenshot above)

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 , I left one code comment and found a few layout issues:

tags are not very visible in dark mode, looks like the background color did not switch:

on some devices, the vertical alignment is too high or too low (see examples). We might consider positioning the container view in UnifiedProloguePages relative to the screen size. I tried something like this in #16187

iPad Pro 9.7 iOS 13.5	iPhone SE 2nd gen iOS 14.4

[Gio2018]

Nice!

[frosty]

@Gio2018 I've fixed the dark mode issue, but let's tackle the spacing of all the views altogether in a future PR.

[Gio2018]

Works great in dark mode.

A few optional minor adjustments to the icons might be made based on the designs, and yes let's address the positioning separately.