BUG: cannot send hotline reply to non-subscribers

• SYMPTOM: when trying to send a hotline reply to a non-subscriber, admins get an invalid message id error ("Sorry, the hotline message identifier #${messageId} has expired or never existed.`" (even though there is in fact a message id for the given combination of sender phone number and channel
• CAUSE: the code for generating hotline replies assumes that the sender must have membership on the channel (which it does so that it can use the membership's language for the message header)
• FIX: provide a fallback default membership (with default language) to use for generating hotline reply headers in the case in which a sender is not a member on the channel