Switch between a project's GitLab boards

A GitLab project can have more than one issue board, and plenty do — one for the team's day-to-day, one for a release, one for a particular slice of the backlog. Until now GitGumbo picked one and that was the board you got.

There is now a board switcher on the board itself, listing that project's boards alphabetically. Switching re-reads the columns from the board you picked, so a board defined around milestones and a board defined around workflow labels each render on their own terms rather than being forced through one shared column definition.

This replaced the GitLab assignee filter that used to sit in that spot. The assignee filter has not gone away — it moved into the board's filter controls, where it sits with the rest of the filtering rather than being the one filter that got its own permanent slot.