Quadrat: Another fix for alignments

[scruffian]

Changes proposed in this Pull Request:

This is another attempt at #3747

The approach is to add left and right padding to paragraph blocks when they are inside a full width container, or when they are inside a container which inherits layout and the screen is smaller than the container.

To test:

• Add a paragraph block inside the post and check that when the page is narrow the content doesn't touch the sides
• Add a paragraph block inside a wide block and check that when the page is narrower than the wide width, the content doesn't touch the sides
• Add a paragraph block inside a full block and check that the content doesn't touch the sides

Related issue(s):

#3747

[scruffian]

Closing in favour of #3953