Create avatar component

Migrate existing or create new avatar component in gitlab-ui.

Design documentation to be added via https://gitlab.com/gitlab-org/design.gitlab.com/issues/129. Documented avatar sizes can be found at gitlab-design#196 (closed)

Should leverage vue-bootstrap where possible, such as rounding corners: https://bootstrap-vue.js.org/docs/components/image. Perhaps it should wrap b-img?