send_email: Improve configurability for outgoing email sender name.

Currently, the sender names for outgoing emails sent by Zulip
are hardcoded. It should be configurable for self-hosted systems.

This commit makes the 'Zulip' part a variable in the following
email sender names: 'Zulip Account Security', 'Zulip Digest',
and 'Zulip Notifications' by introducing a settings variable
'SERVICE_NAME' with the default value as f"{EXTERNAL_HOST} Zulip".

Fixes: #23857

zerver/lib/email_notifications.py

@@ -574,7 +574,9 @@ def do_send_missedmessage_events_reply_in_zulip(
     )
 
     with override_language(user_profile.default_language):
-        from_name: str = _("Zulip notifications")
+        from_name: str = _("{service_name} notifications").format(
+            service_name=settings.INSTALLATION_NAME
+        )
     from_address = FromAddress.NOREPLY
 
     email_dict = {