Enhancement: Improved accessibility for oc-accordion

We improved the accessibility of the oc-accordion component:
- Switch from ul+li to just using divs. A list of accordions is not useful.
- Make sure that collapsed accordions have aria-expanded="false" (attribute vanished from the html before, needs to be a string).
- Remove aria-label from the button as the button already contains all the accessibility hints it needs.

https://github.com/owncloud/owncloud-design-system/pull/1241
