Use GlAvatar in `app/assets/javascripts/groups/components/group_item.vue`

We're rendering some avatars in this component that don't use GlAvatar.

This should be refactored to use GlAvatar.

Existing Avatar code

      <div
        :class="{ 'd-sm-flex': !group.isChildrenLoading }"
        class="avatar-container rect-avatar s32 d-none flex-grow-0 flex-shrink-0"
      >
        <a :href="group.relativePath" class="no-expand">
          <img
            v-if="hasAvatar"
            :src="group.avatarUrl"
            data-testid="group-avatar"
            class="avatar s40"
            :itemprop="microdata.imageItemprop"
          />
          <identicon v-else :entity-id="group.id" :entity-name="group.name" size-class="s40" />
        </a>
      </div>