POST to /typing creates a typing event
Required parameters are 'op' ('start' or 'stop') and 'to' (recipient
emails). If there are multiple recipients, the 'to' parameter
should be a JSON string of the list of recipient emails.
The event created looks like:
{
'type': 'typing',
'op': 'start',
'sender': 'hamlet@zulip.com',
'recipients': [{
'id': 1,
'email': 'othello@zulip.com'
}]
}