SSO Endpoint for discourse

* Adds new app 'discourse' exposing endpoint '/discourse/sso' which performs authentication and calls back the discourse server with user information. (id, username, email, name and avatar)
* Adds new hidden parameter 'next' to auth_view to facilitate the redirect
* Removed unreachable code in views.login
* Removed call to send email to maciej on new user creation (even on dev) - This could be handled better 
* Added 'DISCOURSE_BASE_URL' and 'DISCOURSE_SSO_SECRET' to secret.

I will merge this to the new branch once @mlisbit gives me access.

_DONT PULL UPSTREAM_