Consolidate user avatar Vue logic

Creates 3 vue components:

1. user-avatar-link
2. user-avatar-image
3. user-avatar-svg

and implements them in several places.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/31017

• Port to EE
• Test each implementation manually

EE Compatible Branch: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/1699

app/assets/javascripts/boards/components/issue_card_inner.js

 import Vue from 'vue';
-import userAvatarLink from '../../vue_shared/components/user_avatar/user_avatar_link';
+import userAvatarLink from '../../vue_shared/components/user_avatar/user_avatar_link.vue';
 import eventHub from '../eventhub';
 const Store = gl.issueBoards.BoardsStore;