Move aria-expanded from panel to top level link

[maedi]

aria-expanded should be on the element that expands the dropdown, not the dropdown itself. The attribute is currently on the sub panel div but it needs to be on the a[role="button"] above it.

This recommendation was brought to us by an accessibility consultant and is confirmed in the "Simple dropdown" example on this site: https://www.accessibility-developer-guide.com/examples/widgets/dropdown/

HTML before:

<a role="button"></a>
<div class="sub-nav" aria-expanded="true"></div>

HTML after:

<a role="button" aria-expanded="true"></a>
<div class="panel"></div>

[maedi]

Merged via pull request #66.