Add Universal Analytics support

Adds an admin config setting for selecting a store's analytics script
version.

Implements the analytics.js API in the extended Ga block.
Unlock the session while fetching external downloadable.
Fix IE 8 form labels cannot interact with hidden inputs
Use the first paid invoice _with tax_ for reporting order tax.
Fix input-group button padding to fix with new WDN form sizes
Improve a11y of media gallery for product images.
Improve a11y of standard category links
Add invisible submit button to product form to appease pa11y
Use a button to avoid crawling alternatve product list mode.
Override Magento product option render class to better support a11y
a11y adjustment for product form elements.
Add a11y patches for configurable products JS and HTML.
Add a labeling title to the buy together checkboxes
More contrast for out of stock