add message_user_path to the sfu profile endpoint

message_user_path(user) returns a path that, when followed, opens the message UI and creates a new message addressed to the destination user. For example, https://canvas.sfu.ca/conversations#7b22757365725f6964223a31322c22757365725f6e616d65223a2247726168616d2042616c6c616e74796e65222c22636f6e746578745f6964223a6e756c6c7d will address a new message to me.

I want to allow a user to send a message to an instructor via the course outlines LTI. There doesn't appear to be a Canvas API method to return the message_user_path so I've added it to the SFU profile API.