Added a generic .entry-content-child class to be used (extended) by
anything that needs to apply the content child margins.
Leveraged this .entry-content-child from a new Premium Content selector in
order for the appropraite vertical margins to be applied to premium
content.
The paragraph elements inside of Premium Content blocks weren't
included in the entry-content margin definitions. This ensures
that any paragraph (including those in Premium Content blocks)
have the appropriate styles assigned.
- Makes .entry-content 100% wide to let the box-model work determine .alignfull widths
- .alignwide and default widths now have a fixed width, and a responsive 100% max-width
- Updates utility classes to work with this change
- Improves nested block behaviors and reduces CSS load
- Removes .responsive-max-width utility classes from .entry-content and simialr wrappers that no longer need it
- Renames responsive @extends and @mixins to match Gutenberg pre/suffixes
