Make screen reader announce when API key is copied

Currently screen reader users would click the ‘Copy API key’ button but
not get any feedback. This commit adds an ARIA attribute which tells the
screenreader to announce any changes in to the content of the element
(eg when it changes from showing the API to showing the text ‘API key
copied to clipboard’).

app/templates/components/api-key.html

@@ -2,7 +2,7 @@
   <h2 class="api-key-name">
     {{ name }}
   </h2>
-  <div data-module="api-key" data-key="{{ key }}" data-thing="{{ thing }}">
+  <div data-module="api-key" data-key="{{ key }}" data-thing="{{ thing }}" aria-live="assertive">
     <span class="api-key-key">{{ key }}</span>
   </div>
 {% endmacro %}