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/request_for_comment.rb

@@ -1,7 +1,20 @@
 class RequestForComment < ActiveRecord::Base
-    before_create :set_requested_timestamp
+  belongs_to :exercise
+  belongs_to :file, class: CodeOcean::File
+  belongs_to :user, polymorphic: true
+
+  before_create :set_requested_timestamp
+
+    def self.last_per_user(n = 5)
+      from("(#{row_number_user_sql}) as request_for_comments").where("row_number <= ?", n)
+    end
 
     def set_requested_timestamp
         self.requested_at = Time.now
     end
+
+    private
+    def self.row_number_user_sql
+      select("id, user_id, exercise_id, file_id, requested_at, created_at, updated_at, user_type, row_number() OVER (PARTITION BY user_id ORDER BY created_at DESC) as row_number").to_sql
+    end
 end