Code Cleanup: Usage of Polymorphic User instead of ExternalUser and InternalUser

Renamed requestor_user_id -> user_id
Index of request_for_comments now returns the n (5?) newest requests per user. Solved via sliding windows in postgres, code added to model.
Added route to /my_request_for_comments/ that shows all requests for the current user.
Changed view from ERB to slim

app/models/comment.rb

@@ -4,4 +4,5 @@ class Comment < ActiveRecord::Base
   attr_accessor :username
 
   belongs_to :file, class: CodeOcean::File
+  belongs_to :user, polymorphic: true
 end