swarm: add contacts to contact list for conversation requests

This will make sure that a contact object is inserted into the
contact list when conversation requests are loaded. This will only
add a pending contact if the contact doesn't already exist.

This will prevent contact avatar look-ups from failing for conv
requests after reloading the client, and make sure the registered
name look-ups connect to contact model profile updated signals.

Gitlab: #479
Change-Id: I5f5bd28a6584b7ad148dd7e51443e3654ef30a0e