Admin previews and bug fixes

Big thanks to @erquhart, @marcusrelacion and @brendansparrow for their work on this release.

The main feature is that the admin previews are now accurate to the front-end of the site!

Files changed

.eleventy.js
package.json
src/admin/config.yml
src/_includes/layouts/post.njk
src/admin.njk
src/admin/config.yml
src/admin/previews.js
src/admin/util.js
src/scss/_theme.scss 
src/scss/components/_post.scss
src/scss/components/_syntax-highlighting.scss

Filed added

rollup.config.js