info_overlay: Render keyboard shortcuts using info_overlay.

This commits ports the `keyboard_shortcuts.html` file from
using the Django template to handlebars, essentially creating
a new file as `keyboard_shortcuts.hbs` within /static/templates
which is then rendered using info_overlays.js.

Fixes part of #18792.

zerver/tests/test_home.py

@@ -256,8 +256,6 @@ class HomeTest(ZulipTestCase):
             "Exclude messages with topic",
             "Keyboard shortcuts",
             "Loading...",
-            "Narrow to topic",
-            "Next message",
             "Filter streams",
             # Verify that the app styles get included
             "app-stubentry.js",