Full Site Editing: create post_content block [3]

[vindl]

Create a Gutenberg block that will be responsible for displaying the post's content. This is just for preview purposes and we don't need to allow editing in this stage.

This post_content block will later be used in templates to display how the full template including the post/page it contains will look like.

For testing purposes, it would be helpful to allow picking some existing post/page in the block settings.