Added a new base theme to core called Classy

At DrupalCon Austin a consensus was reached (see #2289511: [meta] Results of Drupalcon Austin's Consensus Banana) to remove CSS classes from core preprocess functions and template files. The first step in this endeavor is moving the classes from preprocess functions and into the core Twig templates. (see #2322163: [meta] Consensus Banana Phase 1, move CSS classes from preprocess to twig templates.) The second step will be copying these core template files into a new base theme called Classy, and strip core's version of these templates of all CSS classes. (see #2348543: [meta] Consensus Banana Phase 2, transition templates to the starterkit theme) The only classes that will remain in core are ones needed to maintain basic functionality.

The change in #2329501: Add classy.info.yml to core, set Classy as base theme for Bartik and Seven creates this new base theme and sets it as the base theme for the Bartik and Seven core themes. Classy's main purpose is to store the class-enriched templates, and serve as a base theme for themes that wish to inherit them. It is not intended as a display theme, and will not contain any of its own functionality or styling.

For Themers

Classy should include all the templates required to style UI component with CSS only. The module CSS that relies on these classes should also be contained within Classy.

Set Classy as your base theme

If you prefer to use many of the CSS classes that currently exist in core's markup, set Classy as your base theme. You do not need to wait for all the template changes to set Classy as your base. You may do so as soon as it is available. When templates and their respective CSS classes are moved from core to the Classy theme, your theme will inherit them, maintaining your design.

Do nothing

If you do not rely on the CSS classes that currently exist in core, and do not want them, simply do nothing. As we progress, those classes will disappear from core's markup. The structure of the markup itself should remain the same.

For Developers

You will still be able to add CSS classes using preprocess functions, so you should be largely unaffected. Moving forward, do not assume any CSS classes that your module did not create will be available. If your module currently adds custom CSS or Javascript that relies on core CSS classes, you may need to adjust your code.