keyboard-shortcuts: Document browse back and forward in history.

Documents in help center `/keyboard-shortcuts` and in the app `?`
menu the shortcuts used by browsers for navigating back and forward
through the open tab's history, which are made to work in Zulip.

Also, updates `adjust_mac_shortcuts` to update the shortcut keys
for users with Mac user agents.

Fixes #18542.

frontend_tests/node_tests/common.js

@@ -100,6 +100,7 @@ run_test("adjust_mac_shortcuts mac non-defaults", ({override}) => {
         ["PgUp", "↑"],
         ["PgDn", "↓"],
         ["Ctrl", "⌘"],
+        ["Alt", "⌘"],
         ["X + Shift", "X + Shift"],
         ["⌘ + Return", "⌘ + Return"],
         ["Enter or Backspace", "Enter or Backspace"],
@@ -157,6 +158,7 @@ run_test("adjust_mac_shortcuts mac defaults", ({override}) => {
         ["PgUp", "↑"],
         ["PgDn", "↓"],
         ["Ctrl", "⌘"],
+        ["Alt", "⌘"],
         ["[", "["],
         ["X", "X"],
     ]);

static/js/common.ts

@@ -61,6 +61,7 @@ export function adjust_mac_shortcuts(key_elem_class: string, kbd_elem = true): v
         ["PgUp", "↑"],
         ["PgDn", "↓"],
         ["Ctrl", "⌘"],
+        ["Alt", "⌘"],
     ]);
 
     const fn_shortcuts = new Set(["Home", "End", "PgUp", "PgDn"]);

static/templates/keyboard_shortcuts.hbs

@@ -101,6 +101,14 @@
                     <td class="definition">{{t 'First message' }}</td>
                     <td><span class="hotkey"><kbd>Home</kbd></span></td>
                 </tr>
+                <tr>
+                    <td class="definition">{{t 'Go back through viewing history' }}</td>
+                    <td><span class="hotkey"><kbd>Alt</kbd> + <kbd class="arrow-key">←</kbd></span></td>
+                </tr>
+                <tr>
+                    <td class="definition">{{t 'Go forward through viewing history' }}</td>
+                    <td><span class="hotkey"><kbd>Alt</kbd> + <kbd class="arrow-key">→</kbd></span></td>
+                </tr>
             </table>
         </div>
         <div>

templates/zerver/help/keyboard-shortcuts.md

@@ -67,6 +67,10 @@ below, and add more to your repertoire as needed.
 
 * **Scroll down**: `PgDn`, `J`, or `Spacebar`
 
+* **Go back through viewing history**: `Alt` + `←`
+
+* **Go forward through viewing history**: `Alt` + `→`
+
 ## Narrowing
 
 * **Narrow to next unread topic**: `n`