R1:dffbbaf0a65b

Added isClosed property to maniphest conduit endpoint in order to fix an issue with arcanist when displaying tasks

Summary:
Arcanist is currently displaying all tasks as closed when invoking `arc tasks`.
This is because arcanist is setting the display to closed if there is anything in the `status` property. Adding an isClosed property will allow arcanist to properly display open/closed status on tasks by checking against the isClosed property. The isClosed property will be set according to the closed…