Bug 1036458 - Add Invalid checkbox to Edit Task page

This implements the ability to mark a task as invalid, which will make it unavailable to users. It will also cause any open attempts to be automatically closed.
I considered this, for just the reason you mention, but the intent of this feature is to allow a user to invalidate a task, which is sort of like a soft delete. I think it's more intuitive to a user to check a box that says `Invalid` to invalidate a task, rather than unchecking a box that says `valid` and is already checked. All tasks are assumed to be valid unless manually invalidated.