notedeck : initial postbox for testing

not sure if we want to put this here yet, but it matches the design

and will be useful for testing

Fixes: https://github.com/damus-io/notedeck/issues/110

Suggested-by: Rob

https://github.com/damus-io/notedeck/commit/26c4d90be38f5b12f34d76d591a0a256d5eb100a