Replace CodeHarbourLink .erb wtih .slim templates

- this is consistent with the remaining view architecture

(leoselig/codeocean#2)

app/models/code_harbor_link.rb

@@ -1,2 +1,8 @@
 class CodeHarborLink < ActiveRecord::Base
+  validates :oauth2token, presence: true
+
+  def to_s
+    oauth2token
+  end
+
 end