[YiR] Populate initial most read slide if needed

[tonisevener]

Phabricator: https://phabricator.wikimedia.org/T376069

Notes

This is a bug I found when testing around on Year in Review. In this situation, I had an older app version 7.6.2 on device, displaying Year in Review with the article count. When then running against the main branch, I noticed the most read day slide didn't show as expected.

The reason is here. Since we already had a list of slides persisted, the most read initial slide is never added and this conditional is skipped. So in users who upgrade from 7.6.2 to 7.6.3, they would never see the new slides.

Test Steps

1. Download TestFlight 7.6.2 build. Ensure region is as expected, build up an article count. Go to app Settings and toggle on and off year in review to display the entry point.
2. Confirm you see year in review article read count slide.
3. In this branch, temporarily set WMFRemoteAppConfigCheckInterval = 1 here to force the app to fetch the new remote config.
4. Then run from this branch, confirm most read day slide appears.

[l-olson1214]

LGTM just some test failures!