Create shared user-avatar-list component

**Why?**
We need a component that can expand / collapse a list of avatars for
approval rules table.

See issue https://gitlab.com/gitlab-org/gitlab-ee/issues/1979

app/assets/javascripts/vue_shared/components/user_avatar/user_avatar_list.vue

+<script>
+import { GlButton } from '@gitlab/ui';
+import { sprintf, __ } from '~/locale';
+import UserAvatarLink from './user_avatar_link.vue';
+
+export default {
+  components: {
+    UserAvatarLink,
+    GlButton,
+  },
+  props: {
+    items: {
+      type: Array,
+      required: true,
+    },
+    breakpoint: {
+      type: Number,
+      required: false,
+      default: 10,
+    },
+    imgSize: {
+      type: Number,
+      required: false,
+      default: 20,
+    },
+  },
+  data() {
+    return {
+      isExpanded: false,
+    };
+  },
+  computed: {
+    visibleItems() {
+      if (!this.hasHiddenItems) {
+        return this.items;
+      }
+
+      return this.items.slice(0, this.breakpoint);
+    },
+    hasHiddenItems() {
+      return this.hasBreakpoint && !this.isExpanded && this.items.length > this.breakpoint;
+    },
+    hasBreakpoint() {
+      return this.breakpoint > 0 && this.items.length > this.breakpoint;
+    },
+    expandText() {
+      if (!this.hasHiddenItems) {
+        return '';
+      }
+
+      const count = this.items.length - this.breakpoint;
+
+      return sprintf(__('%{count} more'), { count });
+    },
+  },
+  methods: {
+    expand() {
+      this.isExpanded = true;
+    },
+    collapse() {
+      this.isExpanded = false;
+    },
+  },
+};
+</script>
+
+<template>
+  <div>
+    <user-avatar-link
+      v-for="item in visibleItems"
+      :key="item.id"
+      :link-href="item.web_url"
+      :img-src="item.avatar_url"
+      :img-alt="item.name"
+      :tooltip-text="item.name"
+      :img-size="imgSize"
+    />
+    <template v-if="hasBreakpoint">
+      <gl-button v-if="hasHiddenItems" variant="link" @click="expand"> {{ expandText }} </gl-button>
+      <gl-button v-else variant="link" @click="collapse"> {{ __('show less') }} </gl-button>
+    </template>
+  </div>
+</template>

locale/gitlab.pot

@@ -106,6 +106,9 @@ msgstr ""
 msgid "%{counter_storage} (%{counter_repositories} repositories, %{counter_build_artifacts} build artifacts, %{counter_lfs_objects} LFS)"
 msgstr ""
 
+msgid "%{count} more"
+msgstr ""
+
 msgid "%{count} more assignees"
 msgstr ""
 
@@ -8175,6 +8178,9 @@ msgstr[1] ""
 msgid "should be higher than %{access} inherited membership from group %{group_name}"
 msgstr ""
 
+msgid "show less"
+msgstr ""
+
 msgid "source"
 msgstr ""
 

spec/javascripts/vue_shared/components/user_avatar/user_avatar_list_spec.js

+import { shallowMount, createLocalVue } from '@vue/test-utils';
+import { GlButton } from '@gitlab/ui';
+import { TEST_HOST } from 'spec/test_constants';
+import UserAvatarList from '~/vue_shared/components/user_avatar/user_avatar_list.vue';
+import UserAvatarLink from '~/vue_shared/components/user_avatar/user_avatar_link.vue';
+
+const TEST_IMAGE_SIZE = 7;
+const TEST_BREAKPOINT = 5;
+
+const createUser = id => ({
+  id,
+  name: 'Lorem',
+  web_url: `${TEST_HOST}/${id}`,
+  avatar_url: `${TEST_HOST}/${id}/avatar`,
+});
+const createList = n =>
+  Array(n)
+    .fill(1)
+    .map((x, id) => createUser(id));
+
+const localVue = createLocalVue();
+
+describe('UserAvatarList', () => {
+  let propsData;
+  let wrapper;
+
+  const factory = options => {
+    wrapper = shallowMount(localVue.extend(UserAvatarList), {
+      localVue,
+      propsData,
+      ...options,
+    });
+  };
+
+  const clickButton = () => {
+    const button = wrapper.find(GlButton);
+    button.vm.$emit('click');
+  };
+
+  beforeEach(() => {
+    propsData = { imgSize: TEST_IMAGE_SIZE };
+  });
+
+  afterEach(() => {
+    wrapper.destroy();
+  });
+
+  describe('with no breakpoint', () => {
+    beforeEach(() => {
+      propsData.breakpoint = 0;
+    });
+
+    it('renders avatars', () => {
+      const items = createList(20);
+      propsData.items = items;
+      factory();
+
+      const links = wrapper.findAll(UserAvatarLink);
+      const linkProps = links.wrappers.map(x => x.props());
+
+      expect(linkProps).toEqual(
+        propsData.items.map(x =>
+          jasmine.objectContaining({
+            linkHref: x.web_url,
+            imgSrc: x.avatar_url,
+            imgAlt: x.name,
+            tooltipText: x.name,
+            imgSize: TEST_IMAGE_SIZE,
+          }),
+        ),
+      );
+    });
+  });
+
+  describe('with breakpoint and length equal to breakpoint', () => {
+    beforeEach(() => {
+      propsData.breakpoint = TEST_BREAKPOINT;
+      propsData.items = createList(TEST_BREAKPOINT);
+    });
+
+    it('renders all avatars if length is <= breakpoint', () => {
+      factory();
+
+      const links = wrapper.findAll(UserAvatarLink);
+
+      expect(links.length).toEqual(propsData.items.length);
+    });
+
+    it('does not show button', () => {
+      factory();
+
+      expect(wrapper.find(GlButton).exists()).toBe(false);
+    });
+  });
+
+  describe('with breakpoint and length greater than breakpoint', () => {
+    beforeEach(() => {
+      propsData.breakpoint = TEST_BREAKPOINT;
+      propsData.items = createList(TEST_BREAKPOINT + 1);
+    });
+
+    it('renders avatars up to breakpoint', () => {
+      factory();
+
+      const links = wrapper.findAll(UserAvatarLink);
+
+      expect(links.length).toEqual(TEST_BREAKPOINT);
+    });
+
+    describe('with expand clicked', () => {
+      beforeEach(() => {
+        factory();
+        clickButton();
+      });
+
+      it('renders all avatars', () => {
+        const links = wrapper.findAll(UserAvatarLink);
+
+        expect(links.length).toEqual(propsData.items.length);
+      });
+
+      it('with collapse clicked, it renders avatars up to breakpoint', () => {
+        clickButton();
+        const links = wrapper.findAll(UserAvatarLink);
+
+        expect(links.length).toEqual(TEST_BREAKPOINT);
+      });
+    });
+  });
+});