event: Fix things in new API event types that clearly aren't right.

These aren't yet used anywhere -- they were salvaged a few commits ago
from src/types.js to seed this new file.  So it's natural that there are
places they're wrong or incomplete.

Fix some of those; mark others safely with `empty` to prevent
accidentally using them before they're reviewed and ready.

Author: gnprice

src/api/eventTypes.js

@@ -58,22 +58,22 @@ export type HeartbeatEvent = {|
 
 export type MessageEvent = {|
   ...EventCommon,
-  type: 'message',
+  type: empty, // 'message' -- TODO fill in rest
 |};
 
 export type PresenceEvent = {|
   ...EventCommon,
-  type: 'message',
+  type: 'presence',
   email: string,
-  presence: { [client: string]: ClientPresence },
   server_timestamp: number,
+  presence: { [client: string]: ClientPresence },
 |};
 
 export type UpdateMessageFlagsEvent = {|
   ...EventCommon,
   type: 'update_message_flags',
+  operation: 'add' | 'remove',
+  flag: empty, // TODO fill in
   all: boolean,
-  flag: 'read' | '???',
   messages: number[],
-  operation: 'add' | '???',
 |};