Enhancement: Lazy img loading & accessibility for OcAvatar

- Add lazy loading to OcImg component
- Internalize former dependency vue-avatar into OcAvatar
- Make OcAvatar use OcImg component, using lazy loading
- Change OcAvatar to be a11y compliant (color contrasts, DOM structure)

https://github.com/owncloud/owncloud-design-system/pull/1282
