Make automated announcement at the start of the batch #90
Description
Summary
Chat-bot should make an announcement via the main Bridge street stream on the first day of the batch
Motivation
In the event that some new rc-ers do not receive a direct message from chat-bot inviting them to join, we'd like to have new users still know about the service and have them directly message chatbot to signup.
Hopefully, this will ensure that no new rc-ers will miss an opportunity to signup for coffee chats!