Make form controls more consistent in their variables and modifiers

[sebnitu]

Problem

We currently have a number of form controls as their own individual components and they differ slightly between their modifiers and how they are customized via SCSS and CSS variables.

Solution

We should audit and refactor all of our form control components and ensure they're consistent in their style language, modifiers and SCSS/CSS variable definitions. This should be approached similarly to how we have core form-control sizing variables. Form control components include the following:

• Button
• Input
• Checkbox
• Radio
• Switch
• Menu (should also follow size scale as form controls)

Example of core form-control variables:

$form-control-size: 2.5rem !default;      // 40px
$form-control-size-sm: 1.875rem !default; // 30px
$form-control-size-lg: 3.125rem !default; // 50px

[sebnitu]

Partially addressed in #1950