Mark code-entered invitation accepted after signup

Due to the EnterInvitationCode view redirecting to the AccountRegistrationView, invites were not marked as accepted.

A changelog should not be necessary, as before one of my recent MRs, the invitation actually was accepted and I removed that code.

aleksis/core/forms.py

@@ -664,6 +664,16 @@ class AccountRegisterForm(SignupForm, ExtensibleForm):
                 setattr(person, field, value)
             person.user = user
             person.save()
+        invitation_code = request.session.get("invitation_code")
+        if invitation_code:
+            from invitations.views import accept_invitation  # noqa
+
+            try:
+                invitation = PersonInvitation.objects.get(key=invitation_code)
+            except PersonInvitation.DoesNotExist:
+                raise SuspiciousOperation()
+
+            accept_invitation(invitation, request, user)
         self.custom_signup(request, user)
         setup_user_email(request, user, [])
         return user