Refactor `form--fancy` to be a modifier on the `form` instead of `input`

[Ryan Trimble]

💰 Refactor

Currently the fancy form stylings need to be applied to inputs as .form__field--field. To make this more consistent with other form styles, we should turn this into a modifier for the .form element, such as .form--fancy.

Benefits of refactor

Simpler to apply to forms, keeps styles consistent with other form styles such as:
.form--rounded
.form--bordered
.form--light

Estimates

FE - 0.5