Update Button to only render `<button>`

[huyenltnguyen]

Description

The Button component can render both a button or an a depending on the href prop:

ui/src/button/button.tsx

Lines 175 to 188 in 884b6ca

	if (href && !disabled) {
	return (
	<Link
	className={className}
	href={href}
	download={download}
	target={target}
	ref={ref as React.Ref<HTMLAnchorElement>}
	onClick={onClick}
	{...rest}
	>
	{children}
	</Link>
	);

We designed the component this way to mimic React Bootstrap's Button to help ease the migration process. However, this implementation has caused some confusion and even code smell (code - the Button receives an href, which makes it a link, but the onClick handler has some conditional that can make the link behave like a button).

I think we should update Button to just render a button element. For links that look like a button, we can either:

• Create a ButtonLink component; or
• Add a style or variant prop our Link component (variant could imply functionality differences, so I'm not sure if this prop is appropriate)

  interface LinkProps {
    style?: 'link' | 'button' // 'link' is the default
  }

Others

The button / link split was also mentioned in #23.