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/vue_shared/components/user_avatar/user_avatar_image.vue

+<script>
 import defaultAvatarUrl from 'images/no_avatar.png';
 import UserAvatarSizeMixin from './user_avatar_size_mixin';
@@ -52,16 +53,18 @@ export default {
       return `avatar ${this.tooltipClass} ${this.avatarSizeClass} ${this.cssClasses}`;
     },
   },
-  template: `
-    <img
-      :class="imgCssClasses"
-      :src="src"
-      :style="avatarSizeStylesMapString"
-      :alt="alt"
-      :data-container="tooltipContainer"
-      :data-placement="tooltipPlacement"
-      :title="tooltipText"
-      ref="tooltipTarget"
-    />
-  `,
 };
+</script>
+
+<template>
+  <img
+    :class="imgCssClasses"
+    :src="src"
+    :style="avatarSizeStylesMapString"
+    :alt="alt"
+    :data-container="tooltipContainer"
+    :data-placement="tooltipPlacement"
+    :title="tooltipText"
+    ref="tooltipTarget"
+  />
+</template>