streams: Allow stream admin to update and deactivate streams.

The new Stream administrator role is allowed to manage a stream they
administer, including:
* Setting properties like name, description, privacy and post-policy.
* Removing subscribers
* Deactivating the stream

The access_stream_for_delete_or_update is modified and is used only
to get objects from database and further checks for administrative
rights is done by check_stream_access_for_delete_or_update.

We have also added a new exception class StreamAdministratorRequired.

zerver/lib/exceptions.py

@@ -224,6 +224,18 @@ class OrganizationOwnerRequired(JsonableError):
     def msg_format() -> str:
         return OrganizationOwnerRequired.OWNER_REQUIRED_MESSAGE
 
+class StreamAdministratorRequired(JsonableError):
+    code: ErrorCode = ErrorCode.UNAUTHORIZED_PRINCIPAL
+
+    ADMIN_REQUIRED_MESSAGE = _("Must be an organization or stream administrator")
+
+    def __init__(self) -> None:
+        super().__init__(self.ADMIN_REQUIRED_MESSAGE)
+
+    @staticmethod
+    def msg_format() -> str:
+        return StreamAdministratorRequired.ADMIN_REQUIRED_MESSAGE
+
 class MarkdownRenderingException(Exception):
     pass