Accessibility Improvements: Premium Blocks

[StoneMoney]

Enhances the accessibility of Engaging Networks' premium blocks by adding the following:

• Semantics improvement for pgHeader (Now tagged as a header level 2 with aria tags)
• Focus outline (Uses --focus_outline_color or #086ce7 as a fallback)
• Aria radiogroup
• Stateful selects (Selects for options on premiums, ie. T-Shirt sizes are disabled unless the premium is selected)
• Label-linked selects (Label and select tags are linked to one-another and more accurately describe what the select is for on screen readers)

Demonstration page: https://give.nationalgeographic.org/page/187835/donate/1?mode=DEMO&assets=accessibility-premiums-demo

• Select any gift other than "4Site Placeholder Product", attempt to change the Color/Size of "4Site Placeholder Product" -- ideally you cannot since it's disabled. Also attempt to navigate to the selects via keyboard (tabbing)
• Switch to selecting "4Site Placeholder Product" and change the color/size, upon changing, check that the label for tags are still present on the selects
• Switch to another gift, selects should be disabled once more and unable to navigate to via keyboard (tabbing)
• Switch back and forth from One-Time to Monthly giving and ensure select labels are linked and properly disabled/enabled based on selection.