Gritter Accessibility Changes

Adding in role="alert" to the main gritter item wrapper to allow for screen readers to pick up on the fact that a new item of notice has popped up. 

Also changed the close div to an a tag to give more semantic meaning to that element. I don't love having the javascript void in there, but wanted to avoid having a '#' chained into the url. Just seemed potentially problematic. The tabindex allows for the user to tab to that element first and then close it via keyboard command, although it doesn't address the issue with tabbing over to the element to begin with.