Fix wrong belongs_to :user

- because of polymorphic user model this didn't work
- changed to :internal_user because that's anyway the only type of user we
  want to impoprt from

(leoselig/codeocean#1)

app/models/code_harbor_link.rb

@@ -2,7 +2,9 @@ class CodeHarborLink < ActiveRecord::Base
   validates :oauth2token, presence: true
   validates :user_id, presence: true
 
-  belongs_to :user
+  belongs_to :internal_user, foreign_key: :user_id
+  alias_method :user, :internal_user
+  alias_method :user=, :internal_user=
 
   def to_s
     oauth2token