signups: Use internal_send_stream_message().

We prefer this to internal_send_message().

We are trying to deprecate `internal_send_message`,
which has extra moving parts related to
`extract_recipients` and `Addressee.legacy_build`.

There are two chunks of code that I touch here
that look pretty similar, but I'm not quite
sure they're worth de-duplicating, since they
use different topics and different message
content.

tools/linter_lib/custom_check.py

@@ -390,7 +390,7 @@ python_rules = RuleList(
              # This one in check_message is kinda terrible, since it's
              # how most instances are written, but better to exclude something than nothing
              ('zerver/lib/actions.py', 'stream = get_stream(stream_name, realm)'),
-             ('zerver/lib/actions.py', 'get_stream("signups", admin_realm)'),
+             ('zerver/lib/actions.py', 'return get_stream("signups", realm)'),
          ]),
          'description': 'Please use access_stream_by_*() to fetch Stream objects',
          },

zerver/lib/actions.py

@@ -254,6 +254,10 @@ def realm_user_count_by_role(realm: Realm) -> Dict[str, Any]:
         RealmAuditLog.ROLE_COUNT_BOTS: bot_count,
     }
 
+def get_signups_stream(realm: Realm) -> Stream:
+    # This one-liner helps us work around a lint rule.
+    return get_stream("signups", realm)
+
 def notify_new_user(user_profile: UserProfile) -> None:
     sender_email = settings.NOTIFICATION_BOT
     sender = get_system_bot(sender_email)
@@ -263,11 +267,10 @@ def notify_new_user(user_profile: UserProfile) -> None:
     # Send notification to realm signup notifications stream if it exists
     # Don't send notification for the first user in a realm
     if signup_notifications_stream is not None and user_count > 1:
-        internal_send_message(
+        internal_send_stream_message(
             user_profile.realm,
-            sender_email,
-            "stream",
-            signup_notifications_stream.name,
+            sender,
+            signup_notifications_stream,
             "signups",
             "@_**%s|%s** just signed up for Zulip. (total: %i)" % (
                 user_profile.full_name, user_profile.id, user_count
@@ -278,17 +281,11 @@ def notify_new_user(user_profile: UserProfile) -> None:
     admin_realm = sender.realm
     try:
         # Check whether the stream exists
-        get_stream("signups", admin_realm)
-    except Stream.DoesNotExist:
-        # If the signups stream hasn't been created in the admin
-        # realm, don't auto-create it to send to it; just do nothing.
-        return
-
-    internal_send_message(
+        signups_stream = get_signups_stream(admin_realm)
+        internal_send_stream_message(
             admin_realm,
-        sender_email,
-        "stream",
-        "signups",
+            sender,
+            signups_stream,
             user_profile.realm.display_subdomain,
             "%s <`%s`> just signed up for Zulip! (total: **%i**)" % (
                 user_profile.full_name,
@@ -297,6 +294,11 @@ def notify_new_user(user_profile: UserProfile) -> None:
             )
         )
 
+    except Stream.DoesNotExist:
+        # If the signups stream hasn't been created in the admin
+        # realm, don't auto-create it to send to it; just do nothing.
+        pass
+
 def notify_invites_changed(user_profile: UserProfile) -> None:
     event = dict(type="invites_changed")
     admin_ids = [user.id for user in
@@ -3788,9 +3790,24 @@ def do_create_realm(string_id: str, name: str,
 
     # Send a notification to the admin realm
     signup_message = "Signups enabled"
-    admin_realm = get_system_bot(settings.NOTIFICATION_BOT).realm
-    internal_send_message(admin_realm, settings.NOTIFICATION_BOT, "stream",
-                          "signups", realm.display_subdomain, signup_message)
+    sender = get_system_bot(settings.NOTIFICATION_BOT)
+    admin_realm = sender.realm
+
+    try:
+        signups_stream = get_signups_stream(admin_realm)
+        topic = realm.display_subdomain
+
+        internal_send_stream_message(
+            admin_realm,
+            sender,
+            signups_stream,
+            topic,
+            signup_message
+        )
+    except Stream.DoesNotExist:  # nocoverage
+        # If the signups stream hasn't been created in the admin
+        # realm, don't auto-create it to send to it; just do nothing.
+        pass
     return realm
 
 def do_change_notification_settings(user_profile: UserProfile, name: str,