Feature/invites 3.0 referral endpoint

[monitz87]

This PR implements the /oauth/callback endpoint, which does the following:

1) Fetch the corresponding unified guild invite (create it if it doesn't exist) 2) Create a new referral assignment between the inviter and the invitee 3) Redirect the invitee to the invite url

[monitz87]

I can create something like a CertainReferral model for this, and leave the old Referral model as-is.

[RafaelVidaurre]

That works @monitz87