Add association User has many CodeHarborLink

- add migration tht associates User with CodeHarborLink
- add belongs_to from CodeHarborLink to User
- changed CodeHarborLinkController#index to only serve current user's
  CodeHarborLinks

(leoselig/codeocean#2)

db/migrate/20160204111716_add_user_to_code_harbor_link.rb

@@ -0,0 +1,5 @@
+class AddUserToCodeHarborLink < ActiveRecord::Migration
+  def change
+    add_reference :code_harbor_links, :user, index: true, foreign_key: true
+  end
+end