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)

app/models/code_harbor_link.rb

@@ -1,5 +1,8 @@
 class CodeHarborLink < ActiveRecord::Base
   validates :oauth2token, presence: true
+  validates :user_id, presence: true
+
+  belongs_to :user
 
   def to_s
     oauth2token