Enhancement: Avatar link & count a11y improvements

- wrappend OcAvatarLink in span (was div)
- wrappend OcAvatarCount in span (was div)
- changed the way the OcAvatarLink background color was picked, which improves the color contrast to >4.5 (a11y requirement)

https://github.com/owncloud/owncloud-design-system/pull/1298
