realtime: add create conversation

[techjeffharris]

• ensure that all invitees allow the invitation
• create conversation
• add starter and online invitees to conversation's list of active members
• subscribe active members to the conversation's event feed
• notify active members that the convo has started and they have joined.

A user should be able to start a conversation with a list of one or more invitees if the invitees' "chatRequests" privacy preferences allow. All other chat requests should return an error.

DEPENDS ON
#21
#14