Fire SSO Login to discourse onLogIn on external site

[simon]

@bvdrie72

• If the user is logged in to the forums, it works exactly as expected.
• If the user logs in to paddling.com (but doesn’t go to the forums first) when the user clicks on the link to spawn a new message, it simply opens the forums (without spawning a new message)

You might be able to use the approach outlined here: Compose a new pre-filled personal message via URL. I haven’t tried using it with the return_path set to compose a prefilled PM, but I think it could be made to work.