topic [nfc]: Mark TODO-server where "subject" can become "topic"

Including in messages' edit history, where the server-side change
hasn't yet landed but is expected to soon: zulip/zulip#21309.

Author: gnprice

src/api/messages/sendMessage.js

@@ -9,6 +9,7 @@ export default async (
   params: {|
     type: 'private' | 'stream',
     to: string,
+    // TODO(server-2.0): Say "topic", not "subject"
     subject?: string,
     content: string,
     localId?: number,

src/api/messages/updateMessage.js

@@ -6,6 +6,7 @@ import { apiPatch } from '../apiFetch';
 export default async (
   auth: Auth,
   params: $ReadOnly<{|
+    // TODO(server-2.0): Say "topic", not "subject"
     subject?: string,
     propagate_mode?: boolean,
     content?: string,

src/api/modelTypes.js

@@ -487,6 +487,7 @@ export type MessageEdit = $ReadOnly<{|
   prev_content?: string,
   prev_rendered_content?: string,
   prev_rendered_content_version?: number,
+  // TODO(server-5.0): Say "topic", not "subject"
   prev_subject?: string,
   timestamp: number,
   user_id: UserId,