Add Watchlist onboarding modal

[mazevedofs]

Phabricator: https://phabricator.wikimedia.org/T335609

Notes

• This PR depends on Add Watchlist onboarding component wikipedia-ios-components#20, make sure that PR is merged and the package commit is updated
• This PR adds the onboarding modal to the Watchlist feature

Test Steps

1. Go to account, click on watchlists
2. The onboarding modal should display
3. Click continue to proceed to the watchlist feature
4. Click learn more to go to a special page (NOTE: I'm using a temporary URL here until I get the correct one)
5. Make sure the layout is correct according to the Figma file, and theming works