models/drafts: Add a model for storing Draft messages.

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>

zerver/lib/export.py

@@ -124,6 +124,7 @@ ALL_ZULIP_TABLES = {
     'zerver_defaultstream',
     'zerver_defaultstreamgroup',
     'zerver_defaultstreamgroup_streams',
+    'zerver_draft',
     'zerver_emailchangestatus',
     'zerver_huddle',
     'zerver_message',
@@ -237,6 +238,9 @@ NON_EXPORTED_TABLES = {
     # This is low priority, since users can easily just reset themselves to away.
     'zerver_userstatus',
 
+    # Drafts don't need to be exported as they are supposed to be more ephemeral.
+    'zerver_draft',
+
     # For any tables listed below here, it's a bug that they are not present in the export.
 }