getMessages types: Handle Message being PmMessage | StreamMessage.

In the same basic way as in the previous commit, but with a bit of
added complexity: the function isn't `(Message) => Message`; it's
`(ServerMessage) => Message`.

So, start by expressing how `ServerMessage` relates to `Message`, in
a way that applies uniformly to `PmMessage` and `StreamMessage`.

Then, annotate the function's param by applying that expression to
`M`, which is a subtype of `Message`, and leave the return value as
`M`.

See discussion at
  https://chat.zulip.org/#narrow/stream/243-mobile-team/topic/.60PmMessage.60.2C.20.60StreamMessage.60.20types.3F/near/1126688.

src/api/messages/getMessages.js

@@ -2,7 +2,7 @@
 import type { Auth, ApiResponseSuccess } from '../transportTypes';
 import type { Identity } from '../../types';
 import type { Message, ApiNarrow } from '../apiTypes';
-import type { Reaction, UserId } from '../modelTypes';
+import type { PmMessage, StreamMessage, Reaction, UserId } from '../modelTypes';
 import { apiGet } from '../apiFetch';
 import { identityOfAuth } from '../../account/accountMisc';
 import { AvatarURL } from '../../utils/avatar';
@@ -31,11 +31,15 @@ export type ServerReaction = $ReadOnly<{|
   |}>,
 |}>;
 
-export type ServerMessage = $ReadOnly<{|
-  ...$Exact<Message>,
+// How `ServerMessage` relates to `Message`, in a way that applies
+// uniformly to `Message`'s subtypes.
+type ServerMessageOf<M: Message> = {|
+  ...$Exact<M>,
   avatar_url: string | null,
   reactions: $ReadOnlyArray<ServerReaction>,
-|}>;
+|};
+
+export type ServerMessage = ServerMessageOf<PmMessage> | ServerMessageOf<StreamMessage>;
 
 // The actual response from the server.  We convert the data from this to
 // `ApiResponseMessages` before returning it to application code.
@@ -49,8 +53,10 @@ export const migrateMessages = (
   messages: $ReadOnlyArray<ServerMessage>,
   identity: Identity,
 ): Message[] =>
-  messages.map(message => ({
-    ...message,
+  messages.map(<M: Message>(message: ServerMessageOf<M>): M => ({
+    // (This casting looks unnecessary, but seems to nudge Flow in the
+    // right direction.)
+    ...(message: ServerMessageOf<M>),
     avatar_url: AvatarURL.fromUserOrBotData({
       rawAvatarUrl: message.avatar_url,
       email: message.sender_email,