Back/forward between threads: ⌘[ / ⌘], header buttons, trackpad swipe #1752

There's currently no way to go back to the thread you were just in. Add a history of opened threads, like a browser:

• ⌘[ goes back and ⌘] goes forward (Ctrl on Windows/Linux)
• back and forward buttons in the sidebar header, disabled when there's nowhere to go
• a two-finger horizontal trackpad swipe on the main pane navigates once you lift your fingers