v4: Updated form and button sizing

Fixes #18607 (closed).

Long story short, the funky vertical padding values we had for inputs and buttons were causing fucked up computed heights when the root font-size is changed from 16px to 14px. This PR updates inputs and buttons to:

• Use easier math (e.g., .5rem on default buttons) to more readily support root font-size changes.
• Due to the increased padding, we now use a tighter line-height (1.25 instead of inherited 1.5). This line-height is the same across all input and button sizes.
• Some mixins and size variation styles were cleaned up as well.