Webforms are completely unstyled

As part of this, we should remove CSS classes that target specific webforms by their config ID, such as webform--newsletter-signup. Webforms have the ability to be assigned arbitrary CSS classes, so instead let's just replace that selector with something like mercury-footer--webform, and then rely on site templates to assign that class to any webforms that they might want to place in the footer.

Since we will now have a webform CSS library, how do we want to attach, sounds like we should avoid attaching to a .twig file. Preprocessor?

I think that'd be fine. It's okay to attach it in a Twig file, just not for a specific webform (identified by ID). Putting it in webform.html.twig would be okay.

Ready to merge, won't be shocked if we have to take another pass at tweaks based on real behavior on Drupal side.