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.vue';
 import eventHub from '../eventhub';
 const Store = gl.issueBoards.BoardsStore;
@@ -38,6 +39,9 @@ gl.issueBoards.IssueCardInner = Vue.extend({
       maxCounter: 99,
     };
   },
+  components: {
+    userAvatarLink,
+  },
   computed: {
     numberOverLimit() {
       return this.issue.assignees.length - this.limitBeforeCounter;
@@ -146,23 +150,17 @@ gl.issueBoards.IssueCardInner = Vue.extend({
           </span>
         </h4>
         <div class="card-assignee">
-          <a
-            class="has-tooltip js-no-trigger"
-            :href="assigneeUrl(assignee)"
-            :title="assigneeUrlTitle(assignee)"
+          <user-avatar-link
             v-for="(assignee, index) in issue.assignees"
             v-if="shouldRenderAssignee(index)"
-            data-container="body"
-            data-placement="bottom"
-          >
-            <img
-              class="avatar avatar-inline s20"
-              :src="assignee.avatar"
-              width="20"
-              height="20"
-              :alt="avatarUrlTitle(assignee)"
-            />
-          </a>
+            class="card-assignee js-no-trigger"
+            :href="assigneeUrl(assignee)"
+            :img-alt="avatarUrlTitle(assignee)"
+            :img-src="assignee.avatar"
+            :img-size="20"
+            :tooltip-text="assigneeUrlTitle(assignee)"
+            tooltip-placement="bottom"
+          />
           <span
             class="avatar-counter has-tooltip"
             :title="assigneeCounterTooltip"