Refactor Paragraph margin rules for consistent, non block-gap values

[pbking]

Changes proposed in this Pull Request:

This change:

• Removes the bottom margin from paragraphs
• Adjusts the top-margin of any element that FOLLOWS a paragraph to be the "paragraph gap" size.

Thus the "paragraph gap" rules are as such:

Inside any wp-container,
Any paragraph element that IS NOT THE FIRST thing and
Any element immediately after a paragraph element
use paragraph gap rules

This allows us to define the paragraph gap rules differently than the general block-gap rules and those rules will be followed on both sides of the paragraph.

Before:

(note that: the gap above the top of the group is 30px while the gap below is 1em)
After:

(note that: the gap above and below the group is 1em)

The gap between the bottom button and image remains 30px.

Related issue(s):

Closes: #4851

[MaggieCabrera]

this looks great to me, thank you!