onboarding: Show one-time modal to introduce inbox view.

To improve onboarding experience, this commit adds
a one-time modal which introduces the inbox view.

Users see this one-time modal on visiting the inbox view.

Fixes part of #29073.

zerver/lib/hotspots.py

@@ -83,6 +83,9 @@ ONE_TIME_NOTICES: List[OneTimeNotice] = [
     OneTimeNotice(
         name="visibility_policy_banner",
     ),
+    OneTimeNotice(
+        name="intro_inbox_view_modal",
+    ),
 ]
 
 # We would most likely implement new hotspots in the future that aren't