A11y: Add support for notifying screen readers that the contents of the page have changed

Currently, screen readers have no way of knowing that, for example, the wizard has been advanced when they click "next" on a portion of the new metadata source form. This applies to most content in general in ShibUI, due to it being a "single page app".

Add functionality to ShibUI that notifies screen readers that forms have been advanced or a major piece of page content has been rendered so that users can know when the page is ready for them to input more data.