There should be an indication when a list for boards is added

This is mainly a UX issue that I experienced.

Steps to reproduce

This should be your first time using this, so you have no idea how it works, the UI should guide you.

1. Go to an issue board with many lists. This means that you are in the far left and there is a scrollbar to take you to the right
2. Go to Add list > Assignee list and filter by your name
3. Click on your name

At this point, there's no indication that something happened and I thought this was broken. It wasn't until I added 3 more people and then scrolled to the right that I saw the created lists 😄

What is the expected correct behavior?

There should be an indication when an assignee list for boards is added. I can think of two things:

1. A tooltip is presented, like when we copy to clipboard
2. A checkmark is added next to the name like we do for label lists

I think the second one makes more sense and is consistent with the rest of the UI.