Style the default Posts listing page (MVP)

Style the default 'all posts' listing page (aka wordpress template: archive.php)

In order to use this feature, an admin needs to set the 'posts page' setting in Settings > Reading (https://www-dev.greenpeace.org/international/wp-admin/options-reading.php)

I (suzi) have set up an example on GPI's dev site: https://www-dev.greenpeace.org/international/news-stories/

Requirements

• It should include a listing of all Posts available including all P4 Post Types (should do this by default)
• Use the category page as a guide for the frontend UI.
• Use the same logic as other listing pages to either show pagination or a "load more" button depending on the relevant feature flag.

Tasks

• Check if we can disable the ability to add blocks/content on the page that is set as 'posts page', to minimize editor confusion. UPDATE: WE CAN!
• Check if we can update the warning text that shows up in the yellow box at the top. UPDATE: WE CAN!
• Warning text should be the following: "The content on this page is hidden because this page is being used as your 'All Posts' listing page. You can disable this by un-setting the 'Posts page' here" (here should link to '/wp-admin/options-reading.php')
• Warning text should be translatable string.