fix: Add missing GlAvatar import in GlNewDropdownItem

In this file, src/components/base/new_dropdown/new_dropdown_item.vue, it is using the GlAvatar component but it was never registered:

L79: <gl-avatar v-if="avatarUrl" :size="32" :src="avatarUrl" />

So this MR imports GlAvatar and registers the component so it can be used successfully.

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I noticed this issue because I was trying to use <gl-new-dropdown-item avatar-url="..."> but got this error:

So I'm hoping that fixing this component in gitlab-ui will allow me to use this component successfully in the GitLab app 😀