request: Extract out methods from 'scheduled_messages' to reuse.

This is a prep commit that extracts the following two methods
from '/actions/scheduled_messages' to reuse in the next commit.
* extract_stream_id
* extract_direct_message_recipient_ids

The 'to' parameter for 'POST /typing' will follow the same pattern
in the next commit as we currently have for the 'to' parameter in
'POST /scheduled_messages', so we can reuse these functions.

zerver/lib/recipient_parsing.py

@@ -0,0 +1,31 @@
+from typing import List
+
+import orjson
+from django.utils.translation import gettext as _
+
+from zerver.lib.exceptions import JsonableError
+
+
+def extract_stream_id(req_to: str) -> int:
+    # Recipient should only be a single stream ID.
+    try:
+        stream_id = int(req_to)
+    except ValueError:
+        raise JsonableError(_("Invalid data type for stream ID"))
+    return stream_id
+
+
+def extract_direct_message_recipient_ids(req_to: str) -> List[int]:
+    try:
+        user_ids = orjson.loads(req_to)
+    except orjson.JSONDecodeError:
+        user_ids = req_to
+
+    if not isinstance(user_ids, list):
+        raise JsonableError(_("Invalid data type for recipients"))
+
+    for user_id in user_ids:
+        if not isinstance(user_id, int):
+            raise JsonableError(_("Recipient list may only contain user IDs"))
+
+    return list(set(user_ids))