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"],
     ]);