Add outline to buttons in ms high contrast mode

[spencercanner]

• Add a transparent border to the product tile button and the iframe toggle
• Transparent borders are picked up by high contrast mode to provide an outline around in button
• Although the borders are not visible in non-high contrast mode, the background colour of the product/cart button is visible through the transparent border so buttons will be slightly bigger
  • background-clip could be used to solve this, however it alters how the border-radius appears. Since border-radius is channel-configurable and is tied to a px value, this approach was not taken.

To 🎩 :

• Sanity check that the product button, cart button, and the cart toggle button do not appear visibly different (with the exception that the product/cart buttons might be slightly bigger)
• In high contrast mode, verify that the product button, cart button, and cart toggle all have a white border

Browsers:

• Safari - Mac
• Firefox - Mac
• Chrome - Mac
• Edge - Mac
• Legacy Edge - Windows
• Edge - Windows
• Chrome - Windows
• Firefox - Windows
• Legacy Edge - Windows - High Contrast Mode