Consolidating two existing issues into one, since they're tightly related: #144 and #5

Current issue

Currently, there's a mismatch between the way site padding behaves in the post/page editor and the frontend.

• In the editor, post content has no padding at all, regardless of its assigned alignment (default, wide, full).
• In the front-end, post content has appropriate default padding (since it's added to our default templates), but full-width items do not go to the screen edges.

Editor	Front End

Target appearance

Our ideal solution is:

• Post content has a default padding that matches between the post/page editor and the front end.
• When a top-level item in the post content is assigned to be "full-width" it should go all the way to the screen edges.

Upstream issues/PRs:

These issues would be solved universally with a solution to the following issue:

• Consider a root-level site padding solution that still lets some items go full-width gutenberg#35607
• Add outer padding support to the flow layout gutenberg#35919
• Alternative approach to the layout outer padding gutenberg#36214