Theme: Fix layout style bleed from previews into main theme

[ryelle]

Fixes #513 — Any styles added via gutenberg_get_layout_style will be added to the global block-supports store in the style engine (definitely new in GB 13.9, possibly from WordPress/gutenberg#42880). The pattern previews used to need a workaround to support layout width (see #428), which was generating the styles injected via window.__editorStyles. This function now also has a side effect of adding that rule to the main site too, which is breaking the admin bar.

The fix added here is to use entry-content class on the preview & thumbnail container divs, so that the theme & default styles are correctly applied. Then we can remove the call to gutenberg_get_layout_style. Removing old workarounds 👍🏻

Props kafleg, dd32

Screenshots

Before	After

This also adds the block spacing between the groups ^ which matches the editor UI, so I think that's overall a good thing.

How to test the changes in this Pull Request:

1. View the front end, the admin bar should not appear centered
2. Create or view some patterns with varying widths
3. The default width should be 800px wide, wide should be 1000px wide, and full width should be edge-to-edge
4. Block spacing matches the editor
5. There should be no regressions in style

[adamwoodnz]

Didn't quite get finished with my review this week @ryelle

The admin bar LGTM but haven't tested with many patterns as my test data is a bit limited. Maybe @dd32 has a bit more context on the old workaround and can approve?

[ryelle]

I'm going to merge this then, I haven't see any issues & would like to fix the admin bar.