Creator: Fix padding in editor

[StevenDufresne]

Introducing Twenty Twenty styles into the editor in #417 created some small regressions.

• The padding around the content in the editor is gone
• The first "block" jumps upwards after first interaction.

This is the result of the Twenty Twenty theme overriding margin and padding on some of Gutenberg's classic.scss styles.

These styles are meant to be a fallback for themes that currently don't use theme.json layouts. See comment in css file for more information.

Current Solution

Copy the Gutenberg styles to our stylesheet.

Alternate Approaches

Re-arrange CSS imports, move Gutenberg after Twenty Twenty one

This would most likely introduce other layout issues.

Control Via theme.json

Try this after #420 is merged

Remaining Issues

• The Block list appender ([+]) still jumps up higher than expected because it's absolutely positioned. In the default editor it isn't noticeable because the title styles are different.

Screenshots

Before	After

How to test the changes in this Pull Request:

1. Go to new-pattern
2. Expect to see around the content
3. Resize the window, expect the padding persists.

[StevenDufresne]

Closing in favor of #428's approach.