A followup to #22.

This PR removes our secondary `style-editor-frame.css` file, and consolidates all of our editor styles into `style-editor.css`. Initially, the standard editor stylesheet was limited in scope to individual blocks, which meant we were unable to style the general alignment of the page, as well as items like the page title and default appender.

The scope of `style-editor.css` is being expanded in WordPress/gutenberg#10956 , so we're now able to style those elements in a single stylesheet.

Some minor style tweaks were necessary to make our original styles work in this context, but for the most part this is identical to #22.

Use of [data-align] selectors appears to be here to stay, so no need to point this out in the header.

Since it's no longer used for the editor frame styles.

So that it builds in case we need to edit it later on.