Handling any deviation from default styling of `Button` components
has been painful, and these changes make it an all or nothing affair:
the `Button` component will apply default styling only in the absence
of a `className` prop. When a `className` prop is present, that will
supersede any component default styling.

These changes make button SCSS mixins clearer and easier to reuse and
extend.

`usePrimaryStyle`, `useCompactStyle`, `useInputStyle` props removed
from `Button` in preference of mixins to reduce `Button` complexity and
confusion over CSS precedence.

Components that use `Button`, and their styles, have been updated here
as needed.