Add a "send new message" link in the PM section
Right now we get to have an Inbox and Sent Messages sections but to send a new PM we need to go searching for a comment or topic of a user, click on its profile and then see the "send PM" button.
As an MVP of this feature, a simple form with an input to write down the username and a textarea for the message (that is the same as for comments) should be more than enough. If someone mispell the username, the form can just return a simple error "user not found".