user_status: Add backend changes to support status emoji.

In this commit:

* We update the `UserStatus` model to accept
`AbstractReaction` as a base class so, we can get all the
fields related to store status emoji.

* We update the user status endpoint
(`users/me/status`) to accept status emoji fields.

* We update the user status event to add status emoji
fields.

Co-authored-by: Yash Rathore <33805964+YashRE42@users.noreply.github.com>

zerver/lib/event_schema.py

@@ -1653,6 +1653,9 @@ user_status_event = event_dict_type(
         # force vertical
         ("away", bool),
         ("status_text", str),
+        ("emoji_name", str),
+        ("emoji_code", str),
+        ("reaction_type", str),
     ],
 )
 _check_user_status = make_checker(user_status_event)