Add system to directly open private chat with a user through URL

[AnthyG]

This could enable a ZeroMail-like experience/ link, that could be shared everywhere to open up the private chat with that user.

If the zite then detects that the said URL is set (e.g. ?PC=[USER_CERT_ID], then the following code just needs to be executed, and the chat is open:

page.addPrivateContact(cert_user_id, page.genContactsList)
page.loadPrivateMessages('selected user', true, cert_user_id)
$('#private_recipient').val(cert_user_id)

[AnthyG]

Added with commit b1d9fec