Don’t have copy API key button jump around

It’s annoying that this button moves after you click on it. It’s
happening because the API key is wrapping onto multiple lines.

This commit fixes the height of the container so that it doesn’t reflow
when it has less content in it.

Uses a bit of flexbox to vertically centre the text.

app/assets/javascripts/apiKey.js

@@ -34,6 +34,8 @@
             thing = $component.data('thing');
 
       $component
+        .addClass('api-key')
+        .css('min-height', $component.height())
         .html(states.keyVisible(key, thing))
         .attr('aria-live', 'polite')
         .on(