Add :forget to forget all left rooms

[VAWVAW]

Left rooms are still synced and in the database unless they are forgotten. Maybe :leave should forget rooms by default since we don't show left rooms.

[ulyssa]

Maybe :leave should forget rooms by default since we don't show left rooms.

I could see this going either "forget by default" or adding a :left (or other name to avoid any possible confusion with left/right directions?) . For now, :forget seems like a good start to helping folks at least have a way to clean up.