Add an option in the settings menu at the top of the page beside the search/filter bar to allow organising list items into groups. This is the same dropdown menu that Show/Hide fields (gitlab-org/gitlab#512668) and Expanding/Nesting (gitlab-org/gitlab#512667) settings are contained in. We should be able to organise items into the following groups: * Assignee * Iteration * Label * Milestone * Health status * Custom status * Custom fields (nice to have in v1)