Avatars > avatars can be links > build

Given the goal of moving shared UI components from gitlab-ce to gitlab-ui, gitlab-ui avatar component needs to support the features supported gitlab-ce avatar component. Clicking an avatar can take the user to a different URL. See https://gitlab.com/gitlab-org/gitlab-ce/blob/master/app/assets/javascripts/vue_shared/components/user_avatar/user_avatar_link.vue for more information of gitlab-ce avatar component