Also add a Draft object-to-dictionary conversion method.
The following commits will provide an API around this
model using which our clients can sync drafts across each
other (if they so wish too). As of making this commit, we
haven't finalized exactly how our clients will use this.
See https://chat.zulip.org/#narrow/stream/2-general/topic/drafts
For some of the discussion around this model and in general,
around this feature.
Signed-off-by: Hemanth V. Alluri <hdrive1999@gmail.com>