Initial a11y improvements

[adamjudd]

This PR includes a number of minor changes to improve a11y support in FoxyShop:

• Switching some ID's to classes to remove potential for duplicate ID's if using shortcodes
  • foxyshop_container, productsubmit, foxyshop_main_price, foxyshop_main_product_image
• Updating some styles to improve contrast
• Adding unique ID to select quantity dropdown, as it is for the quantity input
• Adding radiogroup role and aria-labelledby to radio group variations
• Fixes a bug with update_inventory_alert_language() that wasn't correctly updating the name, it now passes the name in as an additional argument

[rijarobinson]

Looks good.