Twenty Twelve: Add styles and support for the new block-based editor.

This update adds styles and theme support related to the new block-based editor to enhance the experience of using it with Twenty Twelve.

These are the specific changes made to this theme:

- Add `blocks.css`, to style blocks on the front end, to make sure they match the theme’s existing HTML element styles.
- Add `editor-blocks.css` to style blocks in the editor, to make sure they match the theme’s existing HTML element styles.
- Add theme support for `editor-styles`, to pull the existing editor stylesheet into the new editor.
- Add theme support for `wp-block-styles`, to load the default block styles on the front end.
- Add theme support for `editor-color-palette`, to load a color palette based on the theme’s color scheme into the block-based editor. 

Props ianbelanger, crunnells, laurelfulford, davidakennedy.

Merges [43795] to trunk.

Fixes #45040.
Built from https://develop.svn.wordpress.org/trunk@44140


git-svn-id: https://core.svn.wordpress.org/trunk@43970 1a063a9b-81f0-0310-95a4-ce76da25c4cd