Enhancement: Improve accessibility of oc-breadcrumb component

In order to enhance accessibility oc breadcrumb has been wrapped into a <nav> tag.
The <ul> element changed to <ol> element, furthermore aria-current=page applies to the last item and
the default home breadcrumb has been removed.
Keyboard navigation has been fixed.

https://github.com/owncloud/owncloud-design-system/pull/1228
