User can create a private message

[eo-lind]

Story

As a user, I should be able to send a message that only a specific user can see

Acceptance Criteria

Given the user is authenticated
When the user precedes a chat message with an @ symbol, and a friend name (e.g. @friend_name)
Then the only person who should see that chat message is the friend

Given the user is authenticated
When the user is viewing the chat messages
And another user, who is a friend, created a private message
Then the only person who should see that chat message is the current user