mute: Show @-mention messages even if stream or topic muted.

This mirrors the web app's behavior.  For the case of a topic mute,
see `messages_filtered_for_topic_mutes` in `message_list_data.js`.

For the case of a stream mute... the web implementation seems to be
more indirect and I haven't tracked down where stream muting gets
applied to the message list, but empirically it does let messages
through when they have an @-mention.

Fixes: zulip#3472

src/chat/__tests__/narrowsSelectors-test.js

@@ -136,6 +136,13 @@ describe('getShownMessagesForNarrow', () => {
     test('message in a stream is muted if the topic is muted and topic matches', () => {
       expect(shown(makeState({ mute: mutes }))).toEqual(false);
     });
+
+    test('@-mention message is never muted', () => {
+      const flags = { ...eg.plusReduxState.flags, mentioned: { [message.id]: true } };
+      expect(shown(makeState({ flags, subscriptions: [] }))).toEqual(true);
+      expect(shown(makeState({ flags, subscriptions: [mutedSubscription] }))).toEqual(true);
+      expect(shown(makeState({ flags, mute: mutes }))).toEqual(true);
+    });
   });
 
   describe('stream narrow', () => {
@@ -154,6 +161,13 @@ describe('getShownMessagesForNarrow', () => {
     test('message muted if topic is muted', () => {
       expect(shown(makeState({ mute: mutes }))).toEqual(false);
     });
+
+    test('@-mention message is never muted', () => {
+      const flags = { ...eg.plusReduxState.flags, mentioned: { [message.id]: true } };
+      expect(shown(makeState({ flags, subscriptions: [] }))).toEqual(true);
+      expect(shown(makeState({ flags, subscriptions: [mutedSubscription] }))).toEqual(true);
+      expect(shown(makeState({ flags, mute: mutes }))).toEqual(true);
+    });
   });
 
   describe('topic narrow', () => {

src/chat/narrowsSelectors.js

@@ -19,6 +19,7 @@ import {
   getMute,
   getStreams,
   getOutbox,
+  getFlags,
 } from '../directSelectors';
 import { getCaughtUpForNarrow } from '../caughtup/caughtUpSelectors';
 import { getAllUsersById, getOwnUserId } from '../users/userSelectors';
@@ -124,13 +125,17 @@ export const getShownMessagesForNarrow: Selector<$ReadOnlyArray<Message | Outbox
     getMessagesForNarrow,
     state => getSubscriptions(state),
     state => getMute(state),
-    (narrow, messagesForNarrow, subscriptions, mute) =>
+    state => getFlags(state),
+    (narrow, messagesForNarrow, subscriptions, mute, flags) =>
       caseNarrow(narrow, {
         home: _ =>
           messagesForNarrow.filter(message => {
             if (message.type === 'private') {
               return true;
             }
+            if (flags.mentioned[message.id]) {
+              return true;
+            }
             const streamName = streamNameOfStreamMessage(message);
             return (
               showStreamInHomeNarrow(streamName, subscriptions)
@@ -143,6 +148,9 @@ export const getShownMessagesForNarrow: Selector<$ReadOnlyArray<Message | Outbox
             if (message.type === 'private') {
               return true;
             }
+            if (flags.mentioned[message.id]) {
+              return true;
+            }
             const streamName = streamNameOfStreamMessage(message);
             return !isTopicMuted(streamName, message.subject, mute);
           }),
@@ -158,12 +166,8 @@ export const getShownMessagesForNarrow: Selector<$ReadOnlyArray<Message | Outbox
         // In a PM narrow, no messages can be in a muted stream or topic.
         pm: _ => messagesForNarrow,
 
-        // This logic isn't quite right - we want to make sure we never hide a
-        // message that has a mention, even if we aren't in the "Mentioned"
-        // narrow. (#3472)  However, it's more complex to do that, and this code
-        // fixes the largest problem we'd had with muted mentioned messages, which
-        // is that they show up in the count for the "Mentions" tab, but without
-        // this conditional they wouldn't in the actual narrow.
+        // In the @-mentions narrow, all messages are mentions, which we
+        // always show despite stream or topic mutes.
         mentioned: _ => messagesForNarrow,
 
         // The all-PMs narrow doesn't matter here, because we don't offer a