portico: Fix up /features.

Replace "PERSISTENCE" feature with "MUTE USER". All feature
tiles now have Help Center links.

templates/zerver/features.html

@@ -193,20 +193,19 @@
             <p>Enjoy the benefits of threaded conversations while
             controlling your audience and privacy.</p>
         </a>
-        <div class="feature-block">
-            <h3>PERSISTENCE</h3>
-            <p>We’re always receiving messages for you, even when
-            you’re logged out or away from your computer.</p>
-        </div>
+        <a class="feature-block" href="/help/mute-a-user" target="_blank" rel="noopener noreferrer">
+            <h3>MUTE USER</h3>
+            <p>Users can mute anyone they'd rather not interact with.</p>
+        </a>
         <a class="feature-block" href="/help/edit-or-delete-a-message" target="_blank" rel="noopener noreferrer">
             <h3>MESSAGE EDITING</h3>
             <p>Don’t worry, you can always fix that typo, either in
             the body of message or its topic.</p>
         </a>
-        <div class="feature-block">
+        <a class="feature-block" href="/help/status-and-availability#typing-notifications" target="_blank" rel="noopener noreferrer">
             <h3>TYPING NOTIFICATIONS</h3>
             <p>Know when other users are composing messages to you.</p>
-        </div>
+        </a>
         <a class="feature-block" href="/help/view-and-edit-your-message-drafts" target="_blank" rel="noopener noreferrer">
             <h3>SAVED DRAFTS</h3>
             <p>Zulip’s drafts make it easy to write longer messages

templates/zerver/help/private-messages.md

@@ -39,3 +39,7 @@ There are several ways to access an existing PM or group PM.
   able to select PMs or group PMs with that user.
 * Open the compose box, and enter in a (list of) users. Type `Ctrl + .` (or
   `Cmd + .` on a mac) to open that conversation.
+
+## Related articles
+
+* [Typing notifications](/help/status-and-availability#typing-notifications)

templates/zerver/help/status-and-availability.md

@@ -85,3 +85,7 @@ the UI will be frozen as the time you enabled this setting.
 Note that because this setting works by making your availability stop
 updating, you'll still appear to other users as active for a few
 minutes after disabling updates to your availability.
+
+## Typing notifications
+
+Zulip displays typing notifications in private messages.