Add the ability to define our own board's backlog & closed list

Description

Proposal

Add the ability to define our own board's backlog & closed list

Overview

Backlog & Done (closed) does not have the same meaning for everyone. Gitlab should not set default backlog & closed list but propose those from the add list button. The board should be empty (no lists) by default.

This allows for everyone to use it's own backlog & closed list definition, but still use the default gitlab backlog & closed lists.

Use cases

A multi-board example: you might have a todo/wip/done for the UX team, another todo/wip/done the for dev team, another for the test team, etc.

The backlog for the dev team is the done list from the UX team, the backlog for the test team is the done list from the dev team. The true closed list might be only visible for the devops team.

Even without multi-board, the backlog is often filtered and comes from a milestone or labeled list.

Feature checklist

Make sure these are completed before closing the issue, with a link to the relevant commit.