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/views/request_for_comments/show.html.erb

@@ -2,12 +2,9 @@
   <h4 class="list-group-item-heading"><%= Exercise.find(@request_for_comment.exercise_id) %></h4>
   <p class="list-group-item-text">
       <%
-         user = nil
-         if (@request_for_comment.user_type == 'InternalUser')
-           user = InternalUser.find(@request_for_comment.requestor_user_id)
-         else
-           user = ExternalUser.find(@request_for_comment.requestor_user_id)
-         end
+
+           user = @request_for_comment.user
+
       %>
     <%= user %> | <%= @request_for_comment.requested_at %>
   </p>