FIX: do not show private group flair on user avatars (#13872)

Meta ref: https://meta.discourse.org/t/visible-flair-for-invisible-groups-is-that-on-purpose/167674

spec/components/user_lookup_spec.rb

@@ -53,4 +53,32 @@ describe UserLookup do
       expect(user_lookup_group.name).to eq("testgroup")
     end
   end
+
+  describe '#flair_groups' do
+    fab!(:group) { Fabricate(:group, name: "flair_group", flair_icon: "icon", visibility_level: Group.visibility_levels[:public], members_visibility_level: Group.visibility_levels[:public]) }
+    fab!(:user2) { Fabricate(:user, flair_group: group) }
+
+    before do
+      @user_lookup = UserLookup.new([user.id, user2.id, nil])
+    end
+
+    it 'returns nil if user_id does not exists' do
+      expect(@user_lookup.flair_groups[0]).to eq(nil)
+    end
+
+    it 'returns nil if user_id is nil' do
+      expect(@user_lookup.flair_groups[nil]).to eq(nil)
+    end
+
+    it 'returns nil if user has no flair group' do
+      expect(@user_lookup.flair_groups[user.id]).to eq(nil)
+    end
+
+    it 'returns group if user has flair group' do
+      user_lookup_group = @user_lookup.flair_groups[user2.id]
+      expect(user_lookup_group).to eq(group)
+      expect(user_lookup_group.name).to eq("flair_group")
+      expect(user_lookup_group.flair_url).to eq("icon")
+    end
+  end
 end