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.
Prakhar Pratyush