Return the same API representation for 'blocks' and 'parts'.

Summary:
Fixes issue #41. For files which have been uploaded but not actually attached
to a message, we simply set thee 'message' attribute to null.

Also remove some outdated code which defined a namespace property for Part
objects -- we now explicitly save the namespace_id in the Block parent class.

Test Plan: curl http://localhost:5555/n/<ns_id>/files

Reviewers: spang

Reviewed By: spang

Differential Revision: https://review.inboxapp.com/D234