Bundled Theme: Ensure parity between the block editor and the front-end in twentynineteen.

Alignments and blocks were not always appearing as they should in the admin.

- Wide alignments sometimes appear only as wide as the text column.
- Full and wide appear centered on the text column
- Full-width blocks sometimes do not take up the full width of the page, and other times they cause a horizontal scroll.
- Wide and full group block children take up the full width of the block by default.

Fixes #49843.
Props kjellr, Joen, poena.

Built from https://develop.svn.wordpress.org/trunk@48602


git-svn-id: https://core.svn.wordpress.org/trunk@48364 1a063a9b-81f0-0310-95a4-ce76da25c4cd