Twenty Fifteen: Fix issues with theme's Gutenberg support.

A handful of issues were missed in Twenty Fifteen's Gutenberg support, added in #45043. This commit includes the following fixes:

• Center text and bullets in list-based widget blocks, when block itself is centered.
• Add a margin to blocks that are aligned to the left or to the right, to create space.
• Include non "image" variations of the Cover block classes .wp-block-cover and .wp-block-cover-text in the styles.
• Add spacing underneath the Media & Text block, but make sure the final paragraph in the block doesn't add too much space.
• Simplify selectors used to set editor width to just .wp-block.
• Prevent margin from theme's editor-style.css from interfering with editor alignment.

Props pento, davidakennedy.
Fixes #45244.